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I am working with the AD8222 dual package instrumentation amplifier from Analog Devices. I want to build a compact EMG system so I am trying to use this chip since there are not a lot of dual IAs on the market. I need 4 channels eventually.

I have the chip working on a dual supply, and I tested it with a sine wave and the amplitude increases based on the resistor. I tried building the system using a VS- as GND, VS+ as the supply voltage and a voltage divider for the reference voltage. I tested two values of gain resistor (499ohm = 100 gain and 4k which is about 10) and two values for the voltage divider (1k and 10k) to see if the impedance was changing anything.

The output of the amplifier is always the voltage divider value. If I change VS+ to 5V the output jumps to 2.5V. I have tested an input signal of 100 and 200mV with both negative and positive offsets and nothing changes.

The data sheet says the device should work in single supply mode so I am not sure what I am doing wrong. Does anybody have any suggestions of how to change the circuit, or a difference dual package IA that I could test. I really need the single supply since I want to run this system off a battery for a small embedded device.

EDIT: Fixed schematic to reflect the part better.

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Show a schematic. \$\endgroup\$ – Matt Young May 14 '15 at 16:37
  • \$\begingroup\$ Fairly difficult to say anything without seeing the circuit you are building, however this AD8222 Eval Board datasheet has a single supply mode circuit (on the 2nd page) for a differential output. Maybe give that a shot? \$\endgroup\$ – Mewa May 14 '15 at 16:41
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    \$\begingroup\$ Have you tried offsetting your input voltage to whatever your resistor-divider reference is (2.5V for 5V supply)? Fig 19 in the datasheet seems to imply a fairly limited absolute input voltage range which you may be exceeding if your inputs are GND/VS- referenced. \$\endgroup\$ – brhans May 14 '15 at 16:51
  • \$\begingroup\$ 1. The reference "REF" should be low-Z. In other words, you should buffer it. 2. What's \$R_G\$ doing shorting the inputs together? \$\endgroup\$ – Zulu May 14 '15 at 17:01
  • \$\begingroup\$ My mistake. I was confused by the editor tool. I am used to eagle and a different schematic. I never shorted the input pins. \$\endgroup\$ – user3120471 May 14 '15 at 17:08
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Take a peek at figs 7 and 8 in the data sheet. for +/- 5V supply, the common mode input voltage cannot be less than about 4 volts. I suspect that with your 4V supply, and ground as the negative supply, you can't get within 750 mV of ground as a common mode input. While it may operate single supply, it is not rail to rail on the input.

The fact that there is no equivalent to figs 7 and 8 for single supply operation suggests that this is not a very good choice of an instrumentation amp for a single supply emg. I don't know any rail to rail dual package, but suggest you peek at the AD623, which actually works for a common mode voltage a bit BELOW the negative rail.

If your output is stuck at Vref, that would suggest that if it were grounded you'd be stuck at the negative rail. You don't say the size of the sine wave you're using as input, but I suspect it's less than a volt, and below the valid range for common mode input given single supply use. Try putting in a DC 3 volts and see what happens.

Also, that's not how you use a reference terminal on an instrumentation amp. You offset the output stage by providing a LOW IMPEDANCE reference voltage, usually the output of an op amp.

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