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  • Contents

I Description

AD620 is a low-power, high-precision instrumentation amplifier. Based on the information provided by the AD company, this blog introduces the characteristics and typical usage of AD620. Besides, this blog also introduces the application of AD620. The applications are mainly about photoelectric detection, ultrasonic testing, etc.

Figure 1. AD620

I Description

II Introduction

III AD620 Pinout and Working Principle

IV AD620 Typical Usage

V AD620 Application

5.1 Application in Photoelectric Detection

5.2 Application in Ultrasonic Testing

VI Conclusion

FAQ

Ordering & Quantity

II Introduction

AD620 is a low-power, high-precision instrumentation amplifier. And it can set the amplification factor from 1 to 1000 with only an external resistor. It is small in size, in an 8-pin SOIC or DIP package; the power supply range is ±2.3V~±18V; the maximum power supply current is only 1.3mA.

AD620 has good DC and AC characteristics. Its maximum input offset voltage is 50μV, the maximum input offset voltage drift is 1μV/℃, and the maximum input bias current is 2.0nA.

  • When G=10, its common-mode rejection ratio is greater than 93dB. The input voltage noise is at 1kHz, the peak-to-peak value of the input voltage noise is 0.28μV in the range of 0.1Hz~10Hz, and the input current noise is 
  • When G=1, its gain bandwidth is 120kHz, and the settling time is 15μs.

In general, the characteristics of AD620 can be summarized as follows:

  1. AD620 can ensure the performance indicators required for high-gain precision amplification. For example, low offset voltage, low offset voltage drift, and low noise, etc.;
  2. With only one external resistor, the magnification can be set from 1 to 1000;
  3. Small size, with 8 pins;
  4. Low power consumption and its maximum supply current are 1.3mA.

III AD620 Pinout and Working Principle

  The pin of AD620 is shown as in Fig. 2, its structure diagram is shown as in Fig. 3.

  ad620 pinout

  Figure 2. AD620 Pinout

ad620 structure diagram

  Figure 3. AD620 Structure Diagram

AD620 is a monolithic instrument amplifier. It is developed on the basis of the improvement of the traditional three-op amplifier combination.

The input transistors Q1 and Q2 provide the only bipolar differential input. Due to the internal super β processing, its input offset current is 10 times lower than normal.

Through the feedback of the Q1-A1-R1 loop and the Q2-A2-R2 loop, the collector currents of Q1 and Q2 are kept constant. Thus, the input voltage is equivalent to the two ends of the external resistor Rg. And the differential amplification factor from the input to the A1/A2 output is G=(R1+R2)/Rg+1.

The unity gain subtractor composed of A3 eliminates any common-mode components. And then it produces a single-channel output related to the potential of the REF pin.

The value of Rg also determines the transconductance of the previous stage op amp. When Rg decreases, the amplification factor increases. And the transconductance to the input transistor gradually increases. This has obvious advantages: the increase in the amplification factor increases the open-loop gain. Thus, the error related to gain is reduced. The gain-bandwidth product determined by C1, C2 and the pre-op amp transconductance increases. Thus, the frequency response is improved. The input voltage noise is mainly determined by the collector current of the input transistor and the base resistance. And the input voltage noise is reduced to  .

The internal gain resistors R1 and R2 are accurately determined as 24.7kΩ. In this case, the operational amplifier gain is accurately determined by Rg.

  G=49.4kΩ/Rg+1   or   Rg=49.4kΩ/(G-1)

IV AD620 Typical Usage

(1) The input bias current is the current required to bias the input transistor of the op amp, and it must have a return loop. Therefore, when amplifying an AC-coupled signal source like a transformer, each input point must have a DC path to ground. As shown in Figure 4-6.

  ad620 circuit

  Figure 4. Bias Current Loop with Transformer Coupled Input

ad620 circuit

  Figure 5. Bias Current Loop for Thermocouple Input

ad620 circuit

  Figure 6. Bias Current Loop When AC Coupled Input

(2) All instrument amplifiers rectify the signal outside the channel. If a small signal is amplified, this rectified voltage becomes a DC offset voltage. The structure of AD620 allows a first-level filter to be inserted between the base and emitter of the input transistor to filter out unwanted rectified signals, as shown in Figure 5. RC=1/2πf, f is greater than or equal to the bandwidth of AD620, C≤150pF.

  ad620 circuit diagram

  Figure 7. Primary Filter Principle Diagram

(3) The output voltage of AD620 is related to the reference terminal. Connecting the REF terminal to an appropriate grounding point can solve many grounding problems.

Many data acquisition systems separate the analog ground from the digital ground. How come? Just aiming to isolate the low-level analog signal from the noisy digital environment. The grounding principle is as follows: each independent ground loop minimizes the current flowing from the sensitive point to the ground. These ground loops must be connected together at some point, usually on the ADC.

Let's take a look at Figure 7. The reference terminal 5 of the AD620, the ground terminal of the sample-and-hold AD585. And the ground terminal of the analog power supply are respectively connected to the analog ground terminal of the analog-to-digital converter AD574A. The ground terminal of the digital power supply is connected with the digital ground of the analog-to-digital converter AD574A. Finally, the analog ground and digital ground are connected to the AD574A.

In many applications, shielded cables are often used to reduce noise interference at the input. Proper drive to the shield can reduce the differential phase shift caused by cable capacitance and stray capacitance. And ensure that the AC common-mode rejection ratio does not drop.

Figure 8 shows the differential shield drive connection.

Figure 9 shows the common-mode shield drive connection.

ad620 circuit

  Figure 8. Differential Shield Drive

ad620 circuit

  Figure 9. Common Mode Shield Drive

V AD620 Application

5.1 Application in Photoelectric Detection

Photoelectric detection is widely used in the industry. The principle of designing a photoelectric detection system is to reduce the total system noise to a minimum. The noise of the system mainly includes detector noise, resistance noise, and operational amplifier noise. Because they are independent of each other, the total noise can be expressed as

  Un(T)=[Un2(D)+Un2(L,F)+Un2(A)]1/2

In the formula:

  • Un2(D)———Detector noise varies with different photodetectors. The appropriate detector should be selected according to the system requirements;
  • Un2(L,F)———Load resistance noise;
  • Un2(A)———Op amp noise.

The noise of AD620 is very small. Therefore, it is often used as the pre-op amplifier of the photoelectric detection circuit. Take the photodiode as the detector as an example to illustrate. Here, suppose the current of the photodiode is ID, then

  ID=ISC+Idark

In the formula:

  • ISC———Photocurrent. It is proportional to the light intensity, which is the effective information detected;
  • Idark———Dark current. It consists of the current on the surface of the diode and the usual PN junction current. It belongs to invalid information in the system.

We need to eliminate the influence of dark current. Two diodes D1 and D2 with the same performance and two resistors R1 and R2 with the resistance value of R are selected to form a bridge. As shown in Figure 10.

 ad620 circuit

  Figure 10. Photoelectric Detection Preamplifier Circuit

When D1 has light and D2 has no light, the input voltage at both ends of the op-amp is (ISC+Idark)-R1-Idark-R=ISC-R1. It is only related to the photocurrent, thus eliminating the influence of the dark current.

5.2 Application in Ultrasonic Testing

In the field of ultrasonic testing, especially when ultrasonic waves propagate in a non-uniform and attenuated medium, the echo after encountering the interface is very weak.

If a general op-amp is used for pre-processing, the noise is often large and effective signals cannot be obtained. Choosing AD620 as the pre-amplifier circuit has a simple connection and low noise. As shown in Figure 11.

 ad620 circuit

  Figure 11. Ultrasonic Detection Receiving Front Circuit

VI Conclusion

AD620 is a high precision instrumentation amplifier. In the application, we also need to pay attention to prevent the blockage of the amplifier. If a strong DC signal is superimposed on the weak signal, we must set up a separation circuit. In this way, the DC signal can be separated.


FAQ

  • What is AD620?

AD620 is a low-cost, high-precision instrumentation amplifier. It only requires an external resistor to set the gain. The gain range is 1 to 10,000.

  • Can I change AD620 to AD623 when making MCU products?

Both AD620 and AD623 are single instrumentation amplifiers, and the pin arrangement is exactly the same.

The main difference is: AD620 must use positive and negative power supplies, AD623 can be a positive and negative power supply or a single power supply.

If the original board is AD620, you can replace it with 623; if the original board is AD623, you may not be able to replace it with 620 (it depends on whether the power supply of the original board circuit is dual power supply or single power supply).

After replacing AD620 and AD623 in single-chip products, the program can work normally without modification.

  • What is the difference between AD620BR and AD620AN?

Their packages are different.

  • What is the output resistance of AD620? How to adjust it?

AD620 is a kind of low power consumption instrument amplifier, its output resistance is about 10K, this is the inherent characteristic of this chip, generally it is difficult to adjust.

If you have requirements for output resistance, you can generally use an external circuit to solve it.

  • Is AD620 a positive phase amplification or a reverse phase amplification?

AD620 is an instrument amplifier, the output voltage is [(Vin+)-(Vin-)]*gain.

If the desired signal is (Vin+)-(Vin-), the gain is positive, which is equivalent to positive amplification.

Conversely, if the desired signal is (Vin-)-(Vin+), the gain is equivalent to negative, which is equivalent to reverse amplification.

  • What is an instrumentation amplifier?

Instrumentation amplifier, an improvement of the differential amplifier, has an input buffer, does not require input impedance matching, so that the amplifier is suitable for measurement and electronic instruments

 

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