Great job, thank you for making this info available.
On the figures itself I have a few questions if you allow me:
1) in the left image I read -30.1dBV signal from the generator. With a differential gain of 32dB, I would expect an output of 1.9dBV. However I read 8.24dBV output signal, or 6dB more as expected ?
2) in the CM image the image at the right, the plot shows a peak at ca. -88dBV, however the figures that are displayed above the plot are resp -80.2dBV and -58.87dBV.
So what should we look at, the -88dBV in the plot, the -80.2dBV or the -58.87dBV.
But even the worst of these three figures mean 67dBV CMRR (8,24dBV minus -58.87dBV), already a fantastic figure and fully in line with expectation.
Hans
1) in fact there are 2 amplifiers and the gain has +6dB for differential amp. Only one single amplifier has aprox 32dB.
You must looking at the FFT graph, at -88dbV. What you see is what you get. Peak-left is from the initial time during the averaging procces. The stable value, the final value is aprox -88dBV.
You must looking at the FFT graph, at -88dbV. What you see is what you get. Peak-left is from the initial time during the averaging procces. The stable value, the final value is aprox -88dBV.
Last edited:
Yes, there are two amps, each having 20log(1+20k/500)= 32.25dB gain, but when connecting them as one differential amp, the gain remains 32.25 dB.
When the -88dBV is the value to look at, CMRR will be 8.24-(-88) = 96dB an extremely good figure, much better than what any intermediate converter ever could have achieved.
Hans
In a diff mode, the source has two opposite signal. In a CM mode, I was pick up only (+) rail and I was joined the (+) and (-) inputs of the amplifier.
Not sure what you mean exactly, but the picture below shows how to connect the input signal in Diff mode and in CM mode.
You can also see in the plot that the Gain in diff mode is still 41 (=1+20k/500), just as it was with a single amp and not 6dB extra.
Hans
Attachments
Thank you very much. CMRR is the ratio between common mode signal from the input and the output of the amplifier without closed loop gain ?
Cmrr is the ratio of the OUTPUT signals between:
1) when an input signal offered between the diff inputs / 2) when the same input signal is offered between signal gnd and the two diff inputs tied together.
When you have done everything correct, this is In your case resp 8.24dBV - (-88dBV)
I don’t understand your closed loop gain question.
Just take a look at the image in my previous posting.
Hans