Features

Calculation really valid value; average correction value; absolute value; providing 200 MV full marking input range (large input with large input marks); Converter (ADC) direct interface; high input impedance: 1012Ω; low input bias current: maximum 25Pa; high accuracy: ± 0.2 mv ± 0.3%of the readings; RMS conversion with a signal peak factors as high as 5; width power supply: ± 2.5 V to ± 6.5V; low power consumption: 25 μA (typical) available current; no exterior decoration to achieve the specified accuracy; AD737 output is negative transmission; AD736 is the positive output output version of the same basic device.

General description

AD737 is a low power consumption, high -precision, single film, and real valid value DC converter. After laser trimming, it provides the maximum error of ± 0.2 mv ± 0.3%of the reading under the input of sine waves. In addition, it maintains high accuracy when measured the wide range of input waveforms (including variable duty -occupying pulses and TRIAC (phase) control sine waves). The low cost and smaller physical dimensions of AD737 make it suitable for improving the performance of non -equal square root precision rectifiers in many applications. Compared with these circuits, AD737 provides higher accuracy at the same or lower cost.

AD737 calculates the average root value of AC AC and DC input voltage, and is coupled by adding input capacitors. In this mode, despite changes in temperature or power voltage, AD737 still analyzes 100 μV RMS or lower input signal levels. For input waveforms with a peak factors of 1 to 3, the peak factors maintain a high accuracy when the full -standard input level is 2.5%or lower.

AD737 does not have an output buffer amplifier, so it greatly reduces the DC offset error occurred at the output end, making the device highly compatible with the high input impedance ADC.

AD737 only requires a power current of 160 μA, so it is optimized for portable multimeter and other battery power supply applications. In the power -off mode, the input current of the backup power supply is often 25 μA.

AD737 has high (10Ω) and low impedance input options. The high -Z -field effect tube inputs the high source impedance input attenuator, and the low impedance (8 kΩ) input accepts a balanced root voltage of up to 0.9 V, and works at the minimum power supply voltage ± 2.5 V. These two inputs can be used for single -end or differential use.

AD737 realizes a 1%reading error bandwidth. For input amplitude from 20 MV RMS to 200 MV RMS, it only consumes 0.72 mW.

AD737 has two performance levels. AD737J and AD737K are running within the commercial temperature range of 0 ° C to 70 ° C. AD737JR-5 in ± 2.5V DC power supply is tested. The AD737A level works at the industrial temperature range -40 ° C to+85 ° C. AD737 has two low -cost, 8 -guided package: PDIP and SOIC U N.

Product Highlights

1. Calculate the average correction, absolute or true square root value of the signal, which has nothing to do with the waveform.

2. AD737 only requires an external component, that is, the average capacitor, which can be measured.

3, 125μW's standby power consumption enables AD737 to apply for battery power supply.

Typical performance features

TA 25 ° C, ± vs ± 5 v (AD737J-5, except, of which ± vs ± 2.5 v) , CAV 33 μF, CC 10 μF, F 1 KHz, sine wave input is applied to pin 2, unless there are other regulations.

Operation Theory

AD737 has four functional partitions: Enter the amplifier , Full -wave rectifier, average square root magnetic core and partial pressure part (see Figure 23). FET input amplifier allows input high impedance buffer input at the pin 2 or input at low impedance wide dynamic range in the pin 1 input. High impedance input, its low input bias current, is an ideal choice to use high impedance input attenuers. The input signal can be a DC coupling or AC coupling to the input amplifier. Unlike other RMS converters, AD737 allows direct and indirect communication coupling of the input terminal. AC coupling is to achieve direct coupling by placing a series of electrical containers between the input signal and the needle 2 (or needle 1), and a series of electric containers are placed between needle 1 and grounding (driving needle 2) to achieve indirect coupling.

Input the output of the amplifier output driver full -wave precision rectifier, and then drive the RMS core. Its core is to use the external average capacitor CAV to provide a basic RMS operation of a square, average, and square root. If there is no CAV, the input signal after correction will pass the unprocessed core, just like the average response connection (see Figure 25). In the average response mode, the average value is performed by a RC posterior filter. The rear filter consists of an 8 -kΩ internal labeling factors (connected between pin 6 and pin 8) and an external average capacitor C composition Essence In the average square -rooted circuit, the additional filtering level reduces any output ripples that the average capacitor is not eliminated.

Finally, the deviation part allows the power loss function. This reduces the idle current of AD737 from 160 Weire to 30 Weire. This function is selected by connecting the pin 3 to the pin 7 (+vs).

AC measurement type

AD737 passed as a flatA response to the converter or as a real square root DC converter to measure the communication signal. As the name suggests, the average response converter calculates the average absolute value of the AC (or AC and DC) voltage or current by performing a full -wave rectification and low -pass filtering of the input signal; this is similar to the average value. Then, the output (DC average level) generated by increasing (or decrease) gain; this proportional factor converts the average reading of DC to the RMS equivalent value of the tested waveform. For example, the average absolute value of the sine wave voltage is 0.636 times that of VPEAK; the corresponding RMS value is 0.707 times of VPEAK. Therefore, for the sine wave voltage, the required ratio factor is 1.11 (0.707 divided by 0.636).

Different from the measurement average. The really effective value measurement is a common language between waveforms, which allows the amplitude of all types of voltage (or current) waveform to compare with each other and compares to DC. RMS is a direct measurement of the power or thermal value of the AC voltage or the power or thermal value of the DC voltage; the heat generated by the AC signal of the 1V RMS in the resistor is the same as the 1V DC signal.

From mathematics, the average root value of the voltage is defined as:

This includes a square signal, taking the average value, and then finding a square root. The effective value converter is a smart rectifier; they provide accurate valid values, regardless of the type of tested waveform. However, when the input signal of the average response converter deviates from its pre -calibrated waveform, it shows a very high error; the size of the error depends on the type of the tested waveform. For example, if the average response converter is calibrated to measure the average square wave value of the sine wave voltage, and then is used to measure the symmetrical square wave or DC voltage, the calculation error of the converter is 11%higher than the real average square wave value (read) (See Table 5). The transmission function of AD737 is:

DC error, output ripple and average error

Figure 24 shows the typical typical of AD737 under the action of sine wave input voltage. Output waveform. Like all devices in the real world, the ideal V V output will never be fully implemented; on the contrary, the output contains DC and AC error components.

As shown in the figure, the DC error is the difference between the average value of the output signal (eliminating all the ripples in the output through external filtering) and the ideal DC output. Therefore, the DC error component is set only by the value of the average capacitance used, and does not require filtering (using very large filter capacitors, CF) to make the output voltage equal to its ideal value. Using enough large CFs can easily eliminate communication error components, that is, output ripples.

In most cases, when selecting the appropriate value for CAV and CF capacitors, the combination size of DC and AC errors must be considered. This combination error represents the maximum uncertainty of the measurement, which is called the average error, which is equivalent to the peak of the output rippleValue plus DC error. As the input frequency increases, the two error components are reduced rapidly. If the input frequency doubles, the DC errors and ripples are reduced to one -quarter and one -half of its original value, respectively, and quickly become insignificant.

AC measurement accuracy and peak factors

When determining the accuracy of AC measurement, the peak factors of the input waveform are usually ignored. The peak factors are defined as the ratio of the amplitude of the peak signal amplitude to the average square root amplitude (peak factors v/v equity square root). Many common waveforms, such as sine waves and triangular waves, have relatively low peak factors (≥2). Other waveforms, such as low -occupying pulse string and silicon waveform, have a high peak factor. These types of waveforms require a long average time constant to average long periods between the average pulse. Figure 10 shows the relationship between the additional error and peak factors of AD737 under different CAV values.

Calculating settlement time

FIG. 18 can be used to approximately estimate when the input level amplitude of AD737 is reduced, the time it takes. The net settlement time required by the RMS converter is the difference between the two extracted from the figure: the initial time minus the final settlement time. For example, consider the following conditions: 33 μF average capacitors, 100 MV initial RMS input levels, and the final (reduced) input level of 1 MV. According to Figure 18, the initial stability time (intersecting 100 MV lines and 33 μF line) is about 80 ms. The stability time corresponding to the new input or final input level of 1 MV is about 8 s. Therefore, the circuit is stabilized to the new value of 80 milliseconds, which is 7.92 seconds.

Note that due to the inherent smoothness of the comparison characteristics of the capacitor/diode combination, this is the total stability time of the final value (rather than the stable time of 1%, 0.1%of the final value). In addition, this picture provides the worst case of solution, because with the increase of the input level, the AD737 solution is very fast.

Application information

RMS measure Keep the rectifier input signal during the calculation process, and its value directly affects the accuracy of the measuring measuring measuring, especially at low frequency. In addition, as the average capacitor is connected through the diode in the average cubes, the average time constant (τAV) increases the index as the input signal decreases. Therefore, reducing the input signal can reduce the error generated due to non -ideal average, but increased the stable time of the databit value of the average square root calculation near the reduction. Therefore, reducing the input value allowed circuit to perform better while increasing the waiting time between measurement (because the average value is increased). When selecting CAV, weighing the calculation accuracy and determination time.

The fast resolution time of the connection through the average response connection

Due to the average shown in FIG. 25The response connection does not use the average capacitor, so its stable time does not change with the input signal level; it is determined only by the CF RC time constant and the internal 8 -kΩ output target resistor.

For certain applications, I hope to be between RMS value to DC conversion and average value to DC conversion. Select. If the CAV is cut off from the average square root magnetic core, the AD737 full -wave rectifier is a high -precision absolute value circuit. A CMOS switch is controlled by logic levels and selects between the average value and the average root root value.

Table 6 provides the practical value of CAV and CF in several common applications.

The input coupling capacitor CC and 8 kΩ internal input fixed -standard resistors are determined to determine 3 DB low frequency attenuation. This frequency, FL, is equal to:

In addition, if the input voltage has a DC offset of more than 100 MV, in addition to the capacitor CC, the communication coupling network needs to be performed at the pin of 2.

A zoom input and output voltage

AD737 is a very flexible device. Through the smallest external circuit, it can be powered by a single polarity or bipolar power supply, and the input and output voltage can be independently expanded to adapt to non -matching input/output equipment. This section introduces some such applications.

Extension or scaling input range

For low power voltage applications, the input voltage is applied to the internal 8 kΩ input pin 1 to extend the maximum peak voltage of the device. AD737 input circuit quasi -difference work, pin 2 has high impedance FET input (non -conversion), pink 1 has low impedance input (reverse, see Figure 25). The internal 8 kΩ resistor is manifested as a voltage-current converter, which is connected to the context node of the feedback circuit around the input amplifier. Because the feedback loop acts on the voltage of the servo and node voltage to match the two places, the maximum peak input voltage increases until the internal circuit is exhausted. For the symmetrical dual power, this is about twice.

Battery operation

All levels of the battery operation are provided by three semi -digital converters, as shown in Figure 27. Alternatively, the external computing amplifier increases flexibility by regulating non -zero -coefficient voltage and providing output zoom and zero offset. When using an external computing amplifier, the output polarity is positive.

FIGThe calculation amplifier used. Note that the combination input resistance (R1+R2+8 kΩ) is matched with the feedback resistance value of R5. In this case, the size of the output DC voltage is equal to the validity value of the AC input. R3 and R4 provide current to offer output to 0 V.

Adjust the output voltage

The output voltage can be marked to the input average root voltage. For example, assuming the use of the average response circuit (full -wave rectifier) convert AD737 into existing applications. The power supply is 12V, the input voltage is 10V AC, and the expected output is 6V DC.

For the sake of convenience, the input resistance of the same combination shown in FIG. 28. Calculate the RMS input current to:

Select the closest value standard 1%resistor, 47.5 kΩ.

Because the power supply is 12V, the R7/R8 division is 6 V, and the combined resistance value (R3+R4) is equal to feedback resistance, or 47.5 kΩ.

AD737 Evaluation Committee AD737-Evalz evaluation board It can be used for experiments or familiar with the average root to DC converter. Figure 32 is a photo of the circuit board; Figure 34 to Figure 37 shows the copper pattern of signals and power plane. The circuit board is designed for multi -purpose applications and can also be used for AD736. Although it is not provided with the board