AD620 IA Module – ElectroSchematics.com

My next project is to build an entry-level brainwave decoder using an inexpensive microcontroller. I decided a simple way of making the front-end interface would be to use a common instrumentation amplifier chip. In order to accomplish that I searched for a low cost, low power instrumentation amplifier IC and found the AD620 from Analog Devices (https://www.analog.com/media/en/technical-documentation/data-sheets/AD620.pdf ). For many years, the AD620 has been the industry-standard, high performance, low cost instrumentation amplifier. The user can program any desired gain from 1 to 1,000 using a single external resistor.

What’s behind the AD620 Module?

However, just before ordering a couple of AD620 ICs, I came across a Chinese AD620 module available for about half the price of a single AD620 IC. There wasn’t much useful information available, but I planned to save a few bucks and go for it anyway.

The module, packed in a neatly sealed plastic bag, arrived Christmas Eve. The packet comes with one AD620 module and a pair of 4-pin male headers. The module has no branding on either side (quite common for most cheap modules). So, this minuscule instrumentation amplifier module is simply another unbranded/generic piece of little electronics coming from an unknown maker. Some factories seem to be copying modules made for their foreign clients, and selling them in China at a lower price without branding!

Below is a quick description of the AD620 module, copied from a Chinese seller’s product page.

This is a High Precision Signal Amplifier Board/Voltage Amplifier Module based upon AD620 Instrumentation amplifier IC. Input voltage range: DC 3~12V, Amplification: Adjustable 1.5~1000 times, Adjustable zero to improve accuracy. Can be used for AC, DC signal amplification.

  • Wide input range – This product adopts AD620 to amplify and can amplify microvolts and millivolts. Compared with LM358 on the market, it has high amplification precision and good linearity. The maximum voltage output range is ±10V.
  • Magnification – The potentiometer is used to amplify the input signal. The magnification is up to 1000 times and can be adjusted only by a potentiometer.
  • Adjustable zero-point – Adjust the zero point by zeroing potentiometer, improve the accuracy, and there will be no zero drift phenomenon to meet customer needs.
  • The negative pressure output – This module uses 7660A negative voltage chip output negative pressure ( -Vin ), which can be supplied to customers to drive other dual power loads.

This is an annotated image of the module (created by me).

This is the schematic of the AD620 module that I got from the internet after placing my order. As you can see, it comprises just three ICs as key parts – AD620 (instrumentation amplifier), L7660 (negative voltage generator), and LM317 (positive voltage regulator).

Unfortunately, I found that my AD620 module is different, so, the above schematic does not fit. The unusual part of the module is one LM358 op amp IC. In the end, I managed to remake a rough schematic drawing (no guarantees on accuracy) with the help of my digital microscope and other assistive tools. Here’s the new ‘homemade’ schematic:

Now it’s clear! This AD620 module holds three common ICs onboard – AD620A, LM358 and L7660/L7660A. Further inspection verifying the seller’s claims can be done by referring respective datasheets of these ICs.

According to the above datasheets:

  • The AD620 is a low cost, high accuracy instrumentation amplifier that requires only one external resistor to set gains of 1 to1000. Furthermore, the AD620 has a wide power supply range (+/- 2.3 V to +/-18 V).
  • The CMOS voltage converter ICL7660 performs supply voltage conversions from positive to negative for an input range of +1.5V to +10.0V resulting in complementary output voltages of -1.5V to -10.0V (And the ICL7660A does the same with an input range of +1.5V to +12.0V resulting in complementary output voltages of -1.5V to -12.0V).
  • The default maximum supply voltage of LM358 dual-operational amplifier IC is 30 V.

One of the steps of this review is the substantiation of quick specifications provided by the seller. In this case, it’s obvious that the AD620 module can be powered from a 3-12VDC power supply source. Further, since a 100K trimpot is used to adjust the gain of the instrumentation amplifier, yes, the promised range (x1.5 to 1000) seems to be okay. Note that the gain of AD620 can be set by simply connecting the right value of resistor (RG) across the pin +RG (pin 8) and the pin –RG (pin 1). The gain equation then is G = (49.4 KΩ / RG) + 1 so that RG = 49.4 KΩ / G-1. See page 10 of TI’s AD620 datasheet (REV. E) for more details (Or page 12 of Rev. H).

Quick test of the AD620 Module

Presumably, the easiest way to check that the AD620 module is in working condition is to do a basic functionality test. First off, power up the device with your preferred power supply, and then apply a dc voltage to the input and check the output (if without the gain resistor installed, you should then just have a gain of 1).

Here are some important things to keep in mind at this time:

  • Before using the AD620 module, simply bridge its S+ and S- signal inputs with a 0Ω resistor and adjust the zero set trimpot (10K) to get 0V output.
  • It’d be better to set the gain for minimum value using the gain set trimpot (100K) when doing the basic functionality test.
  • In order to set a single-input connection, the S-input must be tied to GND. Thereafter you can feed the single-channel voltage/signal through the S+ input.

The very first thing I tried was testing the AD620 module by powering it up with a regulated 5VDC supply. Then I trimmed the ‘idle’ output close to 0V and tuned the gain to the minimum. Finally, I made a basic mV input test and got a sensible outcome. Since I rarely remember to take a picture, the below half-baked breadboard setup is just a lazy snapshot hours before I tried it.

I think it is difficult to get accurate results because there is a chance that even the smallest travel of the cheap gain set trimpot could throw off the consistency of the readings. Another problem is the lack of a reliable onboard voltage stabilizer circuitry, which raises the probability of distressing noise.

The temporal end

This is not an ideal instrumentation amplifier module, but it is better than a single AD820 in-amp chip as its compact arrangement would probably be useful for entry-level/hobby electronics projects. It is probably something I’ll write about in the future.

Moreover, looking for a ready-to-use instrumentation amplifier module really shows there are not many alternatives. While you could make your own module using a prototyping circuit board, as I have done many times before for INA333, it’s not exactly neat or elegant. Overall, this AD620 module seems to be a good fit for simple applications.

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