I am trying out the AD8293G80 as a possible replacement for our current IA. The IA is used to amplify the output of a pressure sensor and pass the output to an A/D. I have hooked the AD8293 up as indicated in the data sheet but the output (either pin 4 or pin 6) reads 5.1 mV and doesn't change for any change in input. The input signal from the pressure sensor varies between 0 and 10 mV. The common mode voltage is about 2.44 V. The power supply voltage is 5V. I tried another AD8293 and got a similar constant output of 5.6 mV. The pressure sensor is working properly as is the A/D. Do you have any suggestions as to what might be wrong?
Yes, here is a partial schematic. The reference voltage is set to about 200 mV but I have tried the circuit with the reference set to ground as well. I am actually using 1500 pf as the cap between pins 5 and 6.
The minimum voltage you can set on the reference terminal is 0.8V above the negative supply. This would explain the behavior you are observing.
I can think of two options for you:
1. You can increase your reference level. However, since you have a 1.2V reference, this means you'll reduce your input range, and since this part has a fixed-gain, you'll need to increase your reference voltage as well to something like 2.048V or 2.5V. You could consider ADR430 or ADR431.
2. If you can't change your reference voltage, you may want to consider a single-supply rail-to-rail instrumentation amplifier, such as AD8226 or AD8420. Even in this case, keep in mind that the output won't go all the way to zero, so you will need to provide some room for that. Also, for AD8226, you'll need to buffer the divider with an opamp before connecting it to the reference pin.
Let me know if you have any additional questions.
Thank you so much for your response. Obviously I did't read the data sheet carefully enough!
The AD8226 looks like it could be a better choice. I am using the 200 mV reference to accommodate sensors with a negative offset voltage so as to avoid zeroing it in a production environment. Is there a better way to do this?
I'm not sure I understand your question. Are you trying to make the reference voltage adjustable?
No, I am just trying to shift the output voltage higher to accommodate the largest possible negative sensor offset so as to avoid any need for zeroing the offset during production.