Op-amp schematic connection questions and DC biasing

Question

In the first stage the signal is converted to single-ended, and it is amplified using a digital potentiometer in the second stage to be able to change the gain with a microcontroller.

In the last stage I have converted the signal back to differential again.

Thd ADC range is 0-3.3 V and the max. sampling frequency is 5.6 MHz, or 3.3 MHz will be used.

The reason for the differential input is because it has 6 to 8 dB better SNR performance. The signal frequency range of interest is 1 kHz to 1.5 MHz or so.

If you can answer each question specifically it would be great.

ADA4807 Datasheet

My questions are:

1. Is this schematic correct in the case of cascade connections and especially the part where the signal converted back to differential again (the last stage)? The gain is adjusted in the second stage, so the first and last stages are just for proper conversion. Please correct any wrong resistance value I selected, if any.

2. What should be the values of R12 and R14? One reference design uses 1 kΩ, another one uses 10 kΩ. Is it important?

3. Does R10 (10 kΩ) have to be the same as R13 (1 kΩ)? Another way of asking is, is the value of R10 important in this conversion?

4. Do I need R9 and R19 before the DC bias voltage connection?

5. SAR ADC recommends an RC filter at the input for filtering and capacitance recharging, so I have placed 20 Ω and 2.7 nF. Is this correct?

Answer 1

I am answering your question based upon what I believe you are apparently trying to accomplish, rather than what you ask directly.

I believe what you are trying to accomplish is to amplify a differential signal, with a variable gain based upon a single variable resistance. The following circuit will do that job:

^{simulate this circuit – Schematic created using CircuitLab}

R1 and R2 should be precision resistors and have the same value. The differential mode gain will be:

$$A_{V(diff)} = 1 + 2\frac{R1}{R{gain}}$$

You may want to put a fixed resistor in series with the variable resistor to prevent the total resistance going to near 0, and the gain being unreasonably high.

The common mode input voltage is "passed through" this circuit. Thus, if the input signals are centered around 1V, instead of 0V, the output signals will also be centered around 1V.