# ADA2200: Relationship beetween inputsignal amplitude and output signal dc voltage

Question asked by XantaMarius on Jan 29, 2016
Latest reply on Feb 17, 2016 by XantaMarius

Hello,

I'm using the ADA2200 to implement a lock-in amplifier function. There for I provide the Rclk signal of it to a pll (phase locked loop) and get a sine wave of the same frequency. This signal is then used for excitation, or in my case I just amplifier it and provided to the input of the ADA2200. So the amplitude and phase shift between Rclk and signal input is constant. The output is a analog, diskret wave, just like expected. The Signal is than routed to a SAR ADC, without low pass filtering. The ADC is programmed to sample when the synco signal is high but only when at first Rclk is going high. I do this to allways get samples on the same time.

I then calculate the average of the sampled data. I sample I and Q data to calculate the magnitude of the two signals.

My question is:

1) Is this correct or do I need a low pass filter? I thought that averaging the data would be sufficient, isn't it?

2) What puzzles me is how can I conclusion from this value (averaged output signal) to the amplitude of the input signal? (That is my main concern)

I read the "Theory of operation" and the "Amplitude Measurement" sections in the datasheet but there are still questions.

E.g. the Vcyclemean is given as:

VCYCLEMEAN = Conversion Gain × VIN(RMS) × sin(θRELθDEL) =1.05 ×VIN(RMS) × sin(θRELθDEL)

3) But how can I get the θDEL information when I'm not measure a zero cyclemean?

4) How is the relationship between the Vcyclemean and the Amplitude of the input signal?

5) What is the "offset corrected cyclemean" ? Do I have just to subtract the Offset (Vss/2)

6) Should I sample OUTP AND OUTN to get the information about the input signal amplitude or is this the case if the input signal itself is a differential signal?

I don't understand the behaver as well. If there is no input signal, the OUTP is Vss/2. But when I apply a signal it seems that this offset is going down on the OUTP pin. Is this reaction ok?