Hi guys,

I want to amplify a signal and "center" it in 1.65V (3.3/2). For voltage reference Im using a simple voltage divider with two 100k resistors and a buffer (with a LM324 opamp). Im using a INA122 (instrumentation amplifier) to amplify and get a biased signal in 1.65V.

For small gains (~10) it works. But I want greater gains (>100) and in that situation Vref starts decreasing (until GND). Why this happens? What can I do to avoid this?

I also tried to use a common inverting amplifier circuit with a LM324 opamp but the Vref also decreases when gain increase.

Thanks!

 

Is the signal DC coupled to the amp?

A schematic drawing of your circuit would be a big help for answering questions on this problem.


Try letting more current flow in your ref divider. Go with 10k instead of 100k.

 

I agree about the 100k to at least 10k - as that node is being pulled by the 100k in the IA.

Also, you're not railing the output are you?

 

I want to amplify a signal and "center" it in 1.65V (3.3/2).

Why not just pick up a voltage reference IC in that value, or at least adjustable to it?

 

Nevermind!

The problem is not with voltage reference. Im also amplifying offsets and before INA122 I have a high pass filter (2nd order butterworth) removing DC component which maybe isn't removing it all. The signal I want to amplify has ~20mVp-p. So thats the problem, I think.

How can I minimize the effects of offsets? How to "center" the signal on 1.65V (for any gain)? My purpose is to use (aprox.) rail-to-rail output and then get a better resolution after ADC.

Maybe changing Vref of INA122 (with help of a MCU) in acordance to selected gain?

Thanks for your help!

 

I would say that a schematic and details are in order.

 

You said you have a highpass filter in the signal path. If your ADC has a high impedance input, just AC couple between the IA and the ADC. This will be your highpass filter.
The advantage of using a voltage divider (instead of a voltage reference) is that your half-scale biasing will track changes in the ADC reference voltage. Use 1% resistors for stability. See attached circuit.
If your ADC does not have high input impedance, this won't work.

 

RonH, I can't use the HP filter after IA because de signal I want to amplify has a large DC component. It has ~1V DC and I want to amplify the AC component that is about 20mVp-p.
But thats a nice idea! I can use that HP filter with bias to compensate the offset that is amplified, and thus repositioning the signal in 3.3V / 2. I'll give it a try!

Later i'll upload the schematic but thanks for the replies until now!

 

RonH, I can't use the HP filter after IA because de signal I want to amplify has a large DC component. It has ~1V DC and I want to amplify the AC component that is about 20mVp-p.
But thats a nice idea! I can use that HP filter with bias to compensate the offset that is amplified, and thus repositioning the signal in 3.3V / 2. I'll give it a try!

Later i'll upload the schematic but thanks for the replies until now!

Why do you need an active HP filter? Can't you AC couple it twice - once in front of the IA, and again in front of the ADC (with the 3.3V/2 bias)?