feature

10-bit ADC with 9ms conversion time; one ( AD7818 ) and four (AD7817) single-ended analog input channels; AD7816 is a dedicated temperature measurement device; on-chip temperature sensor; resolution 0.258 degrees Celsius; 628C error from -408C to +858C; –55 8C to +1258C operating range; wide operating supply range; +2.7V to +5.5V; inherent track and hold; on-chip reference (2.5 V6 1%); overtemperature indicator; end of conversion Automatic power down; low power operation; 4mW throughput at 10 SPS; 40mW throughput at 1ksps; 400mW throughput at 10 kSPS flexible serial interface.

application

Ambient temperature monitoring (AD7816); thermostat and fan control; high-speed microprocessors; temperature measurement and control; ambient temperature data acquisition systems; monitoring (AD7817 and AD7818); industrial process control; automotive; battery charging applications.

General Instructions

The AD7818 and AD7817 are 10-bit, single- and 4-channel A/D converters with on-chip temperature sensors that operate from a single 2.7 V to 5.5 V supply. Each section contains a capacitor DAC-based 9µs successive approximation converter, an on-chip temperature sensor with ±2°C, an on-chip clock oscillator, inherent track-and-hold functionality, and an on-chip reference (2.5 V). The AD7816 is a dedicated device for temperature monitoring in SOIC/μSOIC packages.

The on-chip temperature sensor of the AD7817 and AD7818 can be accessed through Channel 0. When channel 0 is selected and a conversion is initiated, the resulting ADC code at the end of the conversion measures the ambient temperature with a resolution of ±0.25°C. See the Measuring Temperatures section of the datasheet.

The AD7816, AD7817, and AD7818 have a flexible serial interface that easily interfaces with most microcontrollers. The interface is compatible with Intel 8051, Motorola SPI™ and QSPI™ protocols and National Semiconductor MICROWIRE™ protocol. For more information, see the Serial Interface section of this datasheet.

The AD7817 has a 0.15" 16-lead Small Outline Small Outline IC (SOIC), 16-lead, Thin Shrink Small Outline Package (TSSOP), while the AD7816/AD7818 have an 8-lead Small Outline Integrated Circuit (SOIC) and an 8-lead Microminiature Package Outline Integrated Circuit (μSOIC).

Product Highlights

1. These devices have an on-chip temperature sensor that allows accurate measurement of ambient temperature. The measurable temperature range is -55°C to +125°C.

2. The over-temperature indicator is implemented by digitally comparing the ADC code of channel 0 (temperature sensor) with the content of the on-chip over-temperature register. When the predetermined temperature is exceeded, the over temperature indicator pin goes to logic low.

3. The automatic power-down feature enables the AD7816, AD7817, and AD7818 to achieve excellent power performance at lower throughput rates (eg, 40 microwatts at 1 kSPS throughput).

term signal-to-noise ratio

This is the signal-to-noise ratio (noise + distortion) measured at the output of the A/D converter. The signal is the rms amplitude of the fundamental wave. Noise is the rms sum of all non-fundamental signals up to half the sampling frequency (fS/2), except DC. The ratio depends on the number of quantization levels in the digitization process; the more levels, the less quantization noise. The theoretical signal-to-noise ratio (noise + distortion) of an ideal N-bit converter with a sine wave input is given by:

So for a 10-bit converter, that's 62␣dB.

total harmonic distortion

Total Harmonic Distortion (THD) is the ratio of the root mean square sum of harmonics to the fundamental. For the AD7891, the definitions are as follows:

where V1 is the rms amplitude of the fundamental and V2, V3, V4, V5 and V6 are the rms amplitudes of the second to sixth harmonics.

Peak harmonics or spurious noise

Peak harmonic or spurious noise is defined as the ratio of the rms value of the next largest component (up to fS/2, excluding dc) in the ADC output spectrum to the rms value of the fundamental. Typically, the value of this specification is determined by the largest harmonic in the spectrum, but for the part of the harmonic buried in the noise floor, it will be the noise peak.

Intermodulation Distortion

When the input consists of two sine waves of frequencies fa and fb, any active device with nonlinearity will produce distortion products at the sum and difference frequencies of mfa±nfb, where m, n = 0, 1, 2, 3 Wait. An intermodulation term is a term for which neither m nor n is equal to zero. For example, second-order terms include (fa+fb) and (fa-fb), and third-order terms include (2fa+fb), (2fa-fb), (fa+2fb), and (fa-2fb).

The AD7816, AD7817, and AD7818 were tested using the CCIF standard using two input frequencies near the top of the input bandwidth. In this case, the meanings of the second- and third-order terms are different. The second-order term is usually farther away in frequency from the original sine wave, while the third-order term is usually at a frequency close to the input frequency. Therefore, the second-order and third-order terms are specified separately. Intermodulation distortion is calculated according to the THD specification, where it is the ratio of the rms sum of a single distortion product to the rms amplitude of the fundamental in dBs.

Isolation between channels

Inter-channel isolation is a measure of the level of cross-talk between channels. It is measured by applying a full-scale 20␣kHz sine wave signal to one input channel and determining how much that signal is attenuated in each of the other channels. The numbers given are the worst case for all four channels.

Relative accuracy

Relative accuracy or endpoint nonlinearity is the maximum deviation from a straight line through the endpoints of the ADC transfer function.

Differential nonlinearity

This is the difference between the measurement between any two adjacent codes in the ADC and the ideal 1␣LSB change.

offset error

This is the deviation of the first code transition (0000). . . 000) to (0000. . . 001) from the ideal state, which is AGND+1 LSB.

offset error matching

This is the offset error difference between any two channels.

gain error

This is the deviation of the last code transition (1111). . . 110) to (1111. . 111) from the ideal value, which is VREF – 1 LSB, after offset error adjustment.

Gain Error Matching

This is the difference in gain error between any two channels.

Track/Hold Acquisition Time

Track/hold capture time is the time it takes for the output of the track/hold amplifier to reach its final value (within ±1/2 LSB) after the conversion ends (the point at which the track/hold returns to track mode). It also applies when there is a change in the selected input channel, or when there is a step input change in the input voltage applied to the selected VIN input of the AD7817 or AD7818. This means that the user must wait for the duration of the track/hold acquisition time after the conversion ends or after the channel change/step input changes to VIN before starting another conversion to ensure the part is operating to specification.

control byte

The AD7816, AD7817, and AD7818 contain two on-chip registers, the address register and the overtemperature register. These registers can be accessed by performing an 8-bit serial write to the device. The 8-bit word or control byte written to the AD7816, AD7817, and AD7818 is transferred to one of the following two on-chip registers.

address register

If the five msbs of the control byte are logic zeros, then the three lsbs of the control byte are transferred to the address register, see Figure 4. The address register is a 3-bit wide register that selects the analog input channel on which to perform conversions. It is also used to select the temperature sensor and its address is 000. Table 1 shows the options. The internal reference selection connects the input of the ADC to the bandgap reference. When making this selection and starting a conversion, the ADC output should be approximately mid-scale. After power on, the default channel selection is DB2=DB1=DB0=0 (temperature sensor).

Over temperature register

If any of the five msb of the control byte is a logic 1, then the entire 8 bits of the control byte are transferred to the overheat register, see Figure 4. At the end of the temperature conversion, the 8 MSBs of the temperature conversion result (10 bits) and the contents of the overtemperature register (8 bits). If the result of the temperature conversion is greater than the contents of the over temperature register (OTR), the over temperature indicator (OTI) is logic low. The resolution of OTR is 1 degree Celsius. The minimum temperature that can be written to the OTR is -95 degrees Celsius and the maximum temperature is +152 degrees Celsius - see Figure 5. However, the usable temperature range of the temperature sensor is -55°C to +125°C. Figure 5 shows the OTR and how to set the TALARM (the temperature at which the OTI goes low).

For example, to set TALARM to 50°C, OTR=50+103=153 Dec or 10011001 Bin. If the temperature conversion result exceeds 50°C, the OTI will go to logic low. The OTI logic output is reset high at the end of the serial read operation or when the new temperature measurement falls below TALARM. The default power supply for TALARM is 50°C.

circuit information

The AD7817 and AD7818 are single and quad channel, 9 microsecond conversion time, 10-bit A/D converters with on-chip temperature sensor, reference and serial interface logic functions on a microcontroller. The AD7816 has no analog input channels and is only used for temperature measurements. The A/D converter section consists of a traditional successive approximation converter around a capacitor DAC. The AD7816, AD7817, and AD7818 are capable of operating from a +2.7 V to +5.5 V supply, and the AD7817 and AD7818 accept an analog input range of 0 V to +VREF. On-chip temperature sensors allow accurate measurement of ambient device temperature. The operating measurement range of the temperature sensor is -55°C to +125°C. The part requires a reference voltage of +2.5 V, which can be supplied from the part's own internal voltage reference or from an external voltage reference source. The onchip reference is selected by connecting the REFIN pin to analog ground.

Converter Details

A conversion is initiated by pulsing the CONVST input. The conversion clock for this part is generated internally, so no external clock is required except when reading and writing from the serial port. On-chip Track/Hold From track-to-hold mode, the conversion sequence begins on the falling edge of the CONVST signal. At this point, the busy signal goes high and low again after 9 microseconds or 27 microseconds (depending on whether the analog input or temperature sensor is selected) to indicate the end of the conversion process. This signal can be used by the microcontroller to determine when the conversion result should be read. The track/hold capture time for the AD7817 and AD7818 is 400 ns.

Temperature measurements are made by selecting channel 0 of the on-chip MUX and converting on this channel. The conversion on channel 0 takes 27 microseconds to complete. Temperature measurements are described in the Temperature Measurements section of this data sheet.

The on-chip reference is not available to the user, but REFIN can be driven by an external reference (+2.5 V only). The effect of reference tolerances on temperature measurements is discussed in the section Temperature measurement errors due to reference errors.

All unused analog inputs should be connected to a voltage within the nominal analog input range to avoid noise pickup. For minimum power dissipation, unused analog inputs should be connected to AGND.

Typical Wiring Diagram

Figure 6 shows a typical connection diagram for the AD7817. AGND and DGND are connected together on the device for good noise rejection. The busy line is used to interrupt the microcontroller at the end of the conversion process, and the serial interface is implemented using three lines. See the Serial Interface section for more details. An external 2.5 V reference can be connected to the REFIN pin. If using an external reference, a 10µF capacitor should be connected between REFIN and AGND. For applications involving power consumption, automatic power down at the end of conversion should be used to improve power supply performance. See the Shutdown section of the datasheet.

analog input analog input

Figure 7 shows the equivalent circuit for the analog input structure of the AD7817 and AD7818. Two diodes D1 and D2 provide ESD protection for the analog inputs. Care must be taken to ensure that the analog input signal does not exceed the power rails by more than 200 mV. This will cause these diodes to become forward biased and start conducting current to the substrate. These diodes can draw up to 20mA without irreversible damage. Capacitor C2 in Figure 7 is typically around 4pF, mainly due to pin capacitance. Resistor R1 is a lumped element consisting of the on-resistance of the multiplexer and switch. This resistance is typically around 1kΩ. Capacitor C1 is the ADC sampling capacitor, and its capacitance is 3pf.

DC acquisition time

The ADC starts a new acquisition phase at the end of the conversion and ends on the falling edge of the CONVST signal. At the end of the conversion, the settling time is associated with the sampling circuit. This settling time lasts about 100 nanoseconds. During this time, the analog signal on VIN+ is also acquired. Therefore, the minimum capture time required is about 100 nanoseconds.

Figure 8 shows the equivalent charging circuit for the sampling capacitor when the ADC is in the acquisition phase. R2 is the source impedance of the buffer amplifier or resistor network, R1 is the internal multiplexer resistance, and C1 is the sampling capacitor.

During the acquisition phase, the sampling capacitor must be charged to within 1/2 LSB of its final value. The time required to charge the sampling capacitor (TCHARGE) is given by:

For small source impedance values, the settling time (100ns) associated with the sampling circuit is actually the acquisition time of the ADC. For example, the source impedance (R2) is 10π, and the charging time of the sampling capacitor is about 23 nanoseconds. For source impedances of 1kΩ and larger, the charging time becomes significant.

AC acquisition time

In AC applications, it is recommended to always buffer the analog input signal. The source impedance of the driver circuit must be as low as possible to minimize the acquisition time of the ADC. Larger source impedance values will result in reduced THD at high throughput rates.

on-chip reference

The AD7816, AD7817, and AD7818 have an on-chip +1.2V bandgap reference that allows a +2.5V output. The on-chip reference is selected by connecting the REFIN pin to analog ground. This causes SW1 (see Figure 9) to open and the reference amplifier to power up during the conversion process. Therefore, the on-chip reference is not available externally. An external +2.5 V reference can be connected to the REFIN pin. The effect of this is to turn off the on-chip reference circuit and reduce IDD by about 0.25mA.

ADC transfer function

The output encoding of the AD7816, AD7817, and AD7818 is straight binary. The designed transcoding occurs at consecutive integer LSB values (ie 1 LSB, 2 LSB, etc.). The LSB size is =+2.5 V/1024=2.44 mV. The ideal transfer characteristics are shown in Figure 10 below.

temperature measurement

The on-chip temperature sensor can be accessed through multiplexer channel 0 (ie, by writing 0 0 to the channel address register). Temperature is also the default selection at boot. The transmission characteristics of the temperature sensor are shown in Figure 11 below. The result of the 10-bit conversion on channel 0 can be converted to degrees Celsius using the following formula.

For example, if the conversion result on channel 0 is 100000000 (512 Dec), then the ambient temperature is equal to –103°C + (512/4) = +25°C.

Table II below shows some ADC codes at different temperatures.

Temperature measurement error due to reference error

The AD7816, AD7817, and AD7818 are trimmed with a +2.5 V accurate reference voltage to provide the transfer function described earlier. To show the effect of reference tolerance on temperature readings, the temperature sensor transfer function can be rewritten as a function of reference voltage and temperature.

Code (Dec)=([113.3285××]/[q×]0.6646)×1024 where K=Boltzmann constant, 1.38×10-23q=electron charge, 1.6×10-19t=temperature (K)

For example, calculating the ADC code at 25°C

code=([113.3285×298×1.38×10-23]/[1.6×10-19×2.5]–0.6646)×1024=511.5(200 hex)

As can be seen from the expression, the reference error will produce a gain error. This means that at higher temperatures, the temperature measurement errors due to reference errors will be greater. For example, when the reference error is -1%, the measurement error is +2.2 lsb (0.5°C) at -55°C and +16 lsb (4°C) at +125°C.

Self-heating precautions

The AD7817 and AD7818 feature analog-to-digital conversion capable of 100 kSPS throughput. At this throughput rate, the AD7817 and AD7818 will consume from 4 MW to 6.5 MW. Since thermal impedance is associated with integrated circuit packaging, this power dissipation results in increased die temperature. The figure below shows the self-heating effect in a 16-lead SOIC package. Figures 12 and 13 show the self-heating effects of two- and four-layer PCBs. These graphs were generated by assembling a heater (resistor) and temperature sensor (diode) in the package being evaluated. In Figure 12, the heater (6mw) was turned off after 30 seconds. In the first few seconds after the heater is turned on, the PCB has little effect on self-heating. As can be seen more clearly in Figure 13, the heater is turned off after 2 seconds. Figure 14 shows the relative effects of air, fluid, and self-heating in thermal contact with a large heat sink.

These graphs represent worst-case scenarios for self-heating. In all cases, the heater delivered 6 MW to the inside of the unit. This power level is equivalent to an ADC converting continuously at 100 kSPS. The effects of self-heating can be reduced at lower ADC throughput by operating in Mode 2 - see the "Modes of Operation" section. When operating in this mode, on-chip power consumption is significantly reduced, resulting in self-heating effects.

Operating mode

There are two possible modes of operation for the AD7816, AD7817, and AD7818, depending on the state of the CONVST pulse at the end of the conversion.

Mode 1

In this mode of operation, the CONVST pulse is brought high before the end of the conversion (ie, before busy goes low) (see Figure 16). When operating in this mode, a new conversion should not be initiated until 100 ns after the end of the serial read operation. This quiet time is to allow the track/hold to accurately pick up the input signal after a serial read.

Mode 2

When the AD7816, AD7817, and AD7818 operate in Mode 2 (see Figure 17), they automatically power down at the end of a conversion. CONVST is pulled low to initiate a conversion and remains logic low until the end of the conversion. At this point, that is, when BUSY goes low, the device will be powered down. On the rising edge of the CONVST signal, the device is powered up again. In this mode of operation, the AD7816, AD7817, and AD7818 need only be powered up to perform the conversion, resulting in excellent power performance (see the Power and Throughput section).

Power and Throughput

Excellent power performance can be obtained by using automatic power down (mode 2) at the end of the conversion see the operating modes section of the datasheet.

Figure 18 shows how to implement automatic shutdown to obtain the best power supply performance from the AD7816, AD7817, and AD7818. The device operates in Mode 2 with the duration of the CONVST pulse set equal to the power-on time (2 microseconds). As the throughput of the device decreases, the device remains in its powered-off state longer, and the average power consumption decreases accordingly over time.

For example, if the AD7817 is operating in continuous sampling mode at a throughput rate of 10 kSPS, the power consumption is calculated as follows. The power consumption during normal operation is 6mw, VDD=3v. If the power-on time is 2 microseconds and the conversion time is 9 microseconds, it can be said that the AD7817 typically dissipates 6mW in 11 microseconds (worst case) during each conversion cycle. If the throughput is 10 kSPS, the cycle time is 100 microseconds, and the power consumption per cycle is (11/100) × (6 mW) = 600 microwatts (typical).

AD7817 Serial Interface

The serial interface on the AD7817 is a five-wire interface with read and write functions. Data is read from the output register through the output line, and data is written into the control register through the data input line. The part operates in slave mode and requires an externally applied serial clock to the SCLK input to access data in the data registers or to write control bytes. The RD/WR line is used to determine whether data is written to or read from the AD7817. The RD/WR line is set to logic low when data is written to the AD7817, and is set to logic high when data is read from the part, see Figure 20. The serial interface on the AD7817 is designed to allow the part to be connected to systems that provide a serial clock synchronized with serial data, such as 80C51, 87C51, 68HC11, 68HC05, and PIC16Cxx microcontrollers.

read operation

Figure 20 shows from 7817 AD. CS is brought low to enable the serial interface, and RD/WR is set to logic high to indicate that the data transfer is being read serially from the AD7817. The first data bit (DB9) is clocked by the rising edge of RD/WR, and subsequent bits are clocked on the falling edge of SCLK and are valid on the rising edge. 10 bits of data are transferred during a read operation. However, if the full 10 bits of the conversion result are not required, the user may choose to time only 8 bits. If you are reading 10-bit data, you can access serial data in bytes. However, RD/WR must remain high during data transfer operations. Before starting a new data read operation, the RD/WR signal must go low and high again. At the end of a read operation, the DOUT line enters a high-impedance state on the rising edge of CS or the falling edge of RD/WR, whichever occurs first.

write operation

Figure 20 also shows the control byte write operation to the AD7817. The RD/WR input goes low to indicate an imminent serial write to the part. The AD7817 control byte is loaded on the rising edge of the first 8 clock cycles of the serial clock and data on all subsequent clock cycles is ignored. To perform a second sequential write operation, the RD/WR signal must be turned up and down again.

Simplified serial interface

To minimize the number of interconnect lines to the AD7817, the user can connect the CS line to DGND. This is possible if the AD7817 is not sharing the serial bus with another device. It's also possible to tie up raucous and boisterous scenes together. This configuration is compatible with 8051 microcontrollers. The 68HC11, 68HC05 and PIC16Cxx can be configured to operate using a single serial data line. In this way, the number of lines required to operate the serial interface can be reduced to three, namely RD/WR, SCLK and DIN/OUT, see Figure 6.

AD7816 and AD7818 Serial Interface Modes

The serial interface on the AD7816 and AD7818 is a three-wire interface with read and write capabilities. Data is read from the output registers, and control bytes are written to the AD7816 and AD7818 through the input/output lines. The part operates in slave mode and requires an externally applied serial clock to the SCLK input to access data in the data registers or to write control bytes. The RD/WR line is used to determine whether data is written to the AD7816 and AD7818 or read from the AD7818. When data is written to the device, the RD/WR line is set to logic low, and when data is read from the part, the line is set to logic high, see Figure 21. The serial interface on the AD7816 and AD7818 is designed to allow the part to connect to systems that provide a serial clock that is synchronized with serial data, such as 80C51, 87C51, 68HC11, 68HC05, and PIC16Cxx microcontrollers.

read operation

Figure 21 shows the timing diagram for a serial read from the AD7816 and AD7818. RD/WR is set to logic high to indicate that the data transfer is a serial read from the device. When RD/WR is logic high, the DIN/OUT pin becomes a logic output and the first data bit (DB9) appears on the pin. Subsequent bits are clocked on the falling edge of SCLK, starting with the second SCLK falling edge after RD/WR goes high, and valid on the rising edge of SCLK. 10 bits of data are transferred during a read operation. However, if the full 10 bits of the conversion result are not required, the user may choose to time only 8 bits. If 10-bit data is being read, serial data can be accessed in bytes; however, RD/WR must remain high during a data transfer operation. To perform sequential read operations, the RD/WR pin must be set to logic low and high again. At the end of a read operation, the DIN/OUT pin becomes a logic input on the falling edge of RD/WR.

write operation

The control byte write operation to the AD7816 and AD7818 is also shown in Figure 21. The RD/WR input goes low to indicate an imminent serial write to the part. The AD7816 and AD7818 control bytes are loaded on the rising edge of the first 8 clock cycles of the serial clock, and data on all subsequent clock cycles is ignored. To perform sequential writes to the AD7816 or AD7818, the RD/WR pin must again be set to logic high and low.

Dimensions

Dimensions are in inches and (mm).