feature

Two channels in small 4 mm x 4 mm LFCSP; gain setting with one resistor per amplifier (G=1 to 10000 Ω); low noise; frequency at 8nV/√1kHz; ); high precision DC performance (B-grade); 60μV maximum input bias voltage; 0.3μV/°C maximum input offset drift; 1.0 nA maximum input bias current; minimum common mode rejection ratio 126dB (G=100); Excellent AC performance; 140 kHz bandwidth (G=100); 13 µs settling time to 0.001% differential output option (single channel); fully specified adjustable common mode output supply range: ±2.3V to ±8V.

application

Multi-channel data acquisition; ECG and medical devices; industrial process control; Wheatstone bridge sensors; differential drives; high-resolution input analog-to-digital converters; remote sensors.

General Instructions

The AD8222 is a dual-channel, high-performance instrumentation amplifier that requires only one external resistor per amplifier to set the gain from 1 to 10,000.

The AD8222 is the first dual instrumentation amplifier in a small 4mm x 4mm LFCSP. It requires the same board area as a typical single instrumentation amplifier. The smaller package allows for a 2x increase in channel density and lower cost per channel, all without compromising performance.

The AD8222 can also be configured as a single-channel differential output instrumentation amplifier. The differential output provides high noise immunity, which is useful when the output signal must pass through a noisy environment, such as with remote sensors. This configuration can also be used to drive differential input analog-to-digital converters (ADCs). The AD8222 maintains a minimum common-mode rejection ratio of 80 dB to 4 kHz for all grades at G=1. The high common-mode rejection ratio overfrequency allows the AD8222 to reject broadband interference and line harmonics, greatly simplifying filter requirements. At G=1, the typical CMRR drift temperature of the AD8222 is only 0.07 µV/V/°C.

The AD8222 operates on single and dual supplies, requiring only 2.2 mA of maximum supply current for both amplifiers. It is specified over the industrial temperature range -40°C to +85°C and is fully RoHS compliant.

See AD8221 for a single channel version. Table 1. Instrumentation Amplifiers by Category.

Absolute Maximum Ratings

Stresses greater than or equal to the Absolute Maximum Ratings may cause permanent damage to the product. This is a stress rating only; it does not imply functional operation of the product under the conditions described in the operating section of this specification or any other conditions above. Prolonged operation beyond maximum operating conditions may affect product reliability.

Thermal resistance

The θJA values in Table 6 assume a 4-layer JEDEC standard board. For the LFCSP with thermal pad, it is assumed that the thermal pad is soldered to the platform on the PCB board, and the platform is thermally connected to the thermal power plane. The θJC at the exposed pad is 4.4°C/W.

Maximum power consumption

The maximum safe power dissipation of the AD8222 is limited by the on-chip junction temperature (TJ). At about 130°C, the glass transition temperature, plastics change their properties. Even temporarily exceeding this temperature limit can alter the stress of the package on the die, permanently altering the parametric performance of the amplifier. Temperatures exceeding 130°C for extended periods of time can cause loss of function.

Pin Configuration and Functional Description

Typical performance characteristics

theory of operation

Amplifier structure

The two instrumentation amplifiers of the AD8222 are based on the classic 3 op amp topology. Figure 45 shows a simplified schematic of one of the amplifiers. Input transistors Q1 and Q2 are biased at a fixed current. Any differential input signal will force the output voltage of A1 and A2 to change so that the differential voltage also appears on RG. The current flowing through RG must also flow through R1 and R2, resulting in accurate amplification of the differential input signal between the outputs of A1 and A2. Topologically, Q1+A1+R1 and Q2+A2+R2 can be seen as accurate current feedback amplifiers. The common-mode signal and the amplified differential signal are applied to a differential amplifier that rejects the common-mode voltage. The differential amplifier incorporates innovative techniques for low output bias voltage and low output bias voltage drift.

Since the input amplifier uses a current feedback architecture, the gain-bandwidth product of the AD8222 increases with gain, so that the system does not suffer from the expected bandwidth penalty of a voltage feedback architecture at higher gains.

The transfer function of the AD8222 is:

where:

Gain selection

Placing a resistor on the RG terminal sets the gain of the AD8222, which can be calculated by referring to Table 8 or using the following gain equation:

The AD8222 defaults to G=1 when the gain resistor is not used. The tolerance and gain drift of the RG resistor should be added to the AD8222 specification to determine the overall gain accuracy of the system. Gain error and gain drift are kept to a minimum when no gain resistors are used.

Reference terminal

The output voltage of the AD8222 channel is developed with respect to the potential on the corresponding reference terminal. Typically, the reference terminal is connected to ground, but can also be driven with a voltage to offset the output signal. For example, connect a voltage to the reference terminal to level shift the output so that the AD8222 can drive a single supply ADC. Both REF1 and REF2 are protected by ESD diodes and must not exceed +VS or -VS by more than 0.3v.

For best performance, the source impedance of the reference terminal should be kept below 1Ω. As shown in Figure 45, the reference terminal is at one end of the 10 kΩ resistor. Additional impedance at the reference terminal adds to this 10 kΩ resistance and causes amplification of the signal connected to the positive input. Additional RREF amplification can be done by:

Only the positive signal path is amplified; the negative signal path is not affected. This uneven amplification reduces the common-mode rejection ratio of the amplifier.

Packaging Precautions

The AD8222 uses a 4 mm × 4 mm LFCSP. Be careful not to blindly copy the footprint from another 4 mm x 4 mm LFCSP device; the landing pattern may be different. See the Outline Dimensions section to verify the correct dimensions of the PCB symbol.

The AD8222 is available in two package styles, with thermal pad and without thermal pad.

No thermal pad packaging

The AD8222 ships with a package that does not include a thermal pad; it is the package of choice for the AD8222. Unlike chip-scale packaging, where pads limit routing capabilities, the AD8222 package allows routing and vias directly under the chip, so the full space savings of a small LFCSP can be achieved.

Although the package has no metal in the center of the device, the manufacturing process leaves a small piece of bare metal at each corner of the package, as shown in Figure 56 in the Outline Dimensions area. This metal is connected to -VS through the device. Vias should not be placed under exposed metal due to potential short circuits.

Thermal pad packaging

This package is included mainly for legacy reasons. Because the AD8222 dissipates very little power, a thermal pad is not required.

The thermal pad is internally connected to -VS. The pads can be left open, soldered to other unconnected PCB platforms, or soldered to a platform (-VS) connected to the negative supply rail. If pin compatibility with the AD8224 is required, the pad should not be electrically connected to any nets, including -VS.

The soldering process leaves flux and other contaminants on the board. When these contaminants are located between the AD8222 leads and the thermal pad, they can create leakage paths greater than the AD8222 bias current. A thorough cleaning process can remove these contaminants and restore the AD8222's excellent bias current performance.

layout

The AD8222 is a high precision device. To ensure optimum performance at the PC board level, take care to design the board layout. The AD8222 pins are arranged in a logical manner to assist in this task.

Common Mode Rejection Over Frequency

The common-mode rejection of the AD8222 has higher overfrequency than typical amps, which makes it more immune to disturbances such as line noise and its associated harmonics. To maintain this high performance requires a good implementation layout. The input source impedances should be closely matched. The source resistance should be placed close to the input so that it interacts with as little parasitic capacitance as possible.

Parasitics on the RGx pins can also affect CMRR overfrequency. The PCB layout should match the parasitic capacitance of each pin. The traces from the gain setting resistors to the RGx pins should be kept short to minimize parasitic inductance.

refer to

Errors introduced at the reference are fed directly to the output. Take care to tie the REF to the appropriate local ground.

power supply

Use a regulated DC voltage to power the instrumentation amplifier.

Noise on the power pins can adversely affect performance.

The AD8222 has two positive supply pins (Pin 5 and Pin 16) and two negative supply pins (Pin 8 and Pin 13). Although the device operates with only one pin connected per power pair, both pins should be connected for specified performance and optimum reliability.

The AD8222 should be separated with 0.1µF bypass capacitors, one for each supply. The positive supply decoupling capacitor should be placed near pin 16 and the negative supply decoupling capacitor should be placed near pin 8. Each supply should also be separated from a 10µF tantalum capacitor. Tantalum capacitors can be placed further away from the AD8222 and can often be shared by other precision integrated circuits. Figure 47 shows an example layout.

Input Bias Current Return Path

The input bias current of the AD8222 must have a path back to common. When a source (such as a thermocouple) cannot provide a return path, a return path should be created, as shown in Figure 48.

input protection

All terminals of the AD8222 are ESD protected (2 kV, Human Body Model). In addition, the input structure allows for DC overload conditions of approximately 2.5V above the power supply.

input voltage out of rail

For larger input voltages, external resistors should be used in series with each input to limit current during overload conditions. The AD8222 can safely handle 6 mA of continuous current. The limiting resistance can be obtained from:

For applications where the AD8222 experiences extreme overload voltages such as cardiac defibrillators, external series resistors and low leakage diode clips such as the BAV199L, FJH1100 or SP720 should be used.

Differential Input Voltage at High Gain

When operating at high gain, larger differential input voltages can cause more than 6 mA of current to flow into the input. This condition occurs when the differential voltage exceeds the following critical voltages:

This applies to differential voltages of any polarity.

The maximum allowable differential voltage can be increased by placing an input protection resistor in series with each input. The value of each protection resistor should be:

radio frequency interference

RF correction is often an issue when amplifiers are used in applications with strong RF signals. The disturbance can appear as a small DC offset voltage. High-frequency signals can be filtered by a low-pass RC network placed at the input of the instrumentation amplifier, as shown in Figure 49. The filter limits the input signal bandwidth according to the following relationship:

where C≥10C.

Figure 49 shows an example where the differential filter frequency is about 2kHz and the common-mode filter frequency is about 40kHz.

The values of R and CC should be chosen to minimize RFI. Mismatching the R×CC of the positive input and the R×CC of the negative input reduces the common-mode rejection ratio of the AD8222. Using CD 10, which is 10 times larger than CC, reduces mismatch effects and improves performance.

Common Mode Input Voltage Range

The AD8222's 3-op amp structure applies gain and then removes the common-mode voltage. Therefore, the internal nodes in the AD8222 experience a combination of the obtained signal and the common-mode signal. This combined signal can be limited by the voltage source even in the absence of separate input and output signals. Figure 8 and Figure 9 show the allowable common-mode input voltage range for various output voltages, supply voltages, and gains.

application information

Differential output

The differential configuration of the AD8222 has the same excellent dc accuracy specifications as the single-ended output configuration and is recommended for applications in the frequency range from dc to 100 kHz.

The circuit configuration is shown in Figure 50. The differential output specifications in Table 2 and Table 4 apply only to this configuration. The circuit includes an RC filter to maintain loop stability.

The transfer function of the differential output is:

Set common mode voltage

The output common-mode voltage is set by the average of +IN2 and REF2. The transfer function is:

+IN2 and REF2 have different characteristics, making it easy to set the reference voltage for various applications. +IN2 has high impedance but cannot swing to the device's supply rails. REF2 must be driven with low impedance, but can exceed the supply rails by 300 mV.

A common application is to set the common-mode output voltage to the midscale of a differential ADC. In this case, the ADC reference voltage is sent to the +IN2 terminal and ground is connected to the REF2 terminal. This produces a common-mode output voltage of half the ADC's reference voltage.

2-channel differential output using dual op amps

Another differential output topology is shown in Figure 51. 1/2 of the dual OP2177 op amp produces an inverting output, not 1/2 amp. Since the OP2177 is packaged in an MSOP, this configuration allows the creation of dual precision differential outputs in an amplifier with very small board area.

The error of the op amp is common mode to both outputs. Errors from mismatched resistors can also create a common-mode DC offset. Because these errors are common mode, they are likely to be rejected by the next device in the signal chain.

Driving Differential Input ADCs

The AD8222 can be configured in differential output mode to drive differential analog-to-digital converters. Figure 52 illustrates several concepts.

RFI and antialiasing filters

A 1 kΩ resistor, 1000 pF capacitor, and 100 pF capacitor precede the filter circuit in the form of an amplifier that performs many functions. The 1kΩ and 100pf capacitors form a common-mode filter that protects the amplifier from incoming RF signals. Without filtering, these RFI signals can be rectified in-amp. A 1kΩ resistor provides some overvoltage protection. The 1kΩ resistor and 1000pf capacitor make up the 76khz antialiasing filter for the ADC.

Note that the 100 pF capacitors are of the 5% COG/NPO type. These capacitors are well matched over time and temperature, which keeps the common-mode rejection ratio (CMRR) of the system high over frequency.

second antialiasing filter

Place a 1KΩ resistor and 2200 pF capacitor between each AD8222 output and the ADC input. They created a 72 kHz low-pass filter for another stage of anti-aliasing protection.

These four elements also improve distortion performance. The 2200 pF capacitor provides charge to the switched capacitor front end of the ADC, and the 1kΩ resistor protects the AD8222 from driving any sharp current changes. If the application requires a low frequency antialiasing filter and is sensitive to distortion, increase the value of the capacitor instead of the resistor.

The 1kΩ resistor also protects the ADC from overvoltage. Because the AD8222 operates on a wider supply voltage than a typical ADC, the ADC has the potential to be overdriven. Pulsar converters such as the AD7688 do not have this problem. Its input can handle 130 mA of overdrive, which is well above the short-circuit limit of the AD8222. However, other converters are less robust and may require additional protection.

refer to

The ADR435 provides the reference voltage for the ADC and AD8222. Since REF2 on the AD8222 is grounded, the common-mode output voltage is exactly half the reference voltage, exactly what the ADC needs.

Precision Strain Gauge

The low offset and high CMRR overfrequency characteristics of the AD8222 make it ideal for AC and DC bridge measurements. As shown in Figure 53, the bridge can be connected directly to the input of the amplifier.

drive cable

All cables have a certain amount of capacitance per unit length, which varies widely with cable type. The capacitive loading of the cable can cause peaks in the output response of the AD8222. To reduce peaking, use a resistor between the AD8222 and the cable. Since cable capacitance and desired output response vary widely, it is best to determine this resistance empirically. A good starting point is 50Ω.

The AD8222 operates at a low enough frequency that transmission line effects are rarely a problem; therefore, the resistors do not have to be matched to the characteristic impedance of the cable.

Dimensions