Understanding an instrumentation amplifier (AD8221AR)

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1

Hi,

I am hoping to use the instrumentation amplifier (show in the image) but I am unable to find a datasheet. I am wondering if anyone has experience, or a good guess as to how this chip will work?

Typically, an instrument amp will have 2 reference pins to control the amplifier gain, but this doesn't seem to have that?

Screenshot_20251224-104730
Screenshot_20251224-104730806×894 72.4 KB

2

Have you tried searching the forum for ad8221 ?

3

Consult the AD8221 datasheet.

The gain is set by a single resistor connected between pins 2 and 3 of the IC.

Show us a picture of the other side of the PCB that shows the IC, and then we might be able to help better.

It does seem rather strange that an 8 pin IC only has 7 pins brought out to the pins of the module.

4

That amplifier is by Analog Devices. They will have a lot of information on that device and more likely than not, articles on applications and instrumentation amplifiers in general.

What are you hoping to use it for?

5

The gain is fixed at 100.0

image
image720×540 92.6 KB

Requires +/- supplies.

6

This is one of my pet peeves with these arduino modules. It would cost pennies to add a voltage inverter so you could run it from +5v only and they could sell it for a few bucks more.

The just never think about how these things will be used and it's just a race to see who can be cheapest.

7

It looks to me as though the manufacturers of the module have based it on Figure 49 of the datasheet, but with 3 capacitors omitted:

AD8221 Module
AD8221 Module630×528 24.5 KB

8

Here is a pic of the important side of that board. The gain set resistor (marked 68A, 499 R) is connected to pin 3.

Capture
Capture708×704 47.6 KB

9

If you are competent at soldering surface mount components, then you could alter the gain by changing the 499Ω resistor (marked 68A).

AD8221 Gain Selection
AD8221 Gain Selection462×413 31.6 KB

10

REF is not connected to ground.

11

Here is the other side of the board I show in post #5

image
image720×540 95.8 KB

12

You are quite right, I hadn't spotted that on the schematic I showed.

13

Can be used, with limited output swing, on a single supply with Ref connected to VCC/2.

But why would you use this chip with an Arduino.
Leo..

14

Thanks for the replies. I intend to use this module to amplify a pressure sensor which provides a sensitivity of 0.2mV / kPa.

For the operating range I require, this will output a range of 0 mV to 40mV. If the board has a fixed value of 499Ω (G = 100), I think that will be fine for my applications. It will lose a little accuracy as the Uno reads up to 5V (with ADC) but, good enough for my applications.

Thank you everyone.

15

Better to use an HX711 module.
Instrumentation amp that is designed for low voltage and 24-bit A/D, all in one.
Leo..

16

Thank you for your help.
What is the best way to mirror the voltage and provide a negative supply? My current DC power source does not provide any negative voltage lines.

17

That would be a complete waste of your time. 40 mV/ (5 mV/ADC step) = 8 ADC steps for the range. On the other hand, it would be a good learning experience.

18

Tha AD8221 can swing between 0volt and 3.8volt on a bi-polar supply,
which could give you about 778 data points (in theory) with a 10-bit A/D.
Must protect the Arduino pin against negative voltages though with a resistor/diode clamp.
Leo..

19

You can use two 9V batteries like this:

9V-P-N
9V-P-N394×576 33.1 KB

Or if you have two isolated supplies you can connect then in a similar manner.

20

Does the Uno not use a 10-bit ADC, so I would have a step size of about 0.004V? I am not an electrical engineer so I may be mistaken.