Differentail ADC issue on STM32 Board

[Samudra]

Hi Sir,
We are using the STM32H747XIH6 microcontroller in our custom board. We have configured the ADA4930 amplifier in a single-ended to differential configuration with unity gain. The op-amp output is connected to the differential ADC1_IN2 channel (positive and negative inputs).

The op-amp output looks fine, as shown in the attachment. However, we are not able to read the signal using the differential ADC to reconstruct the input sine wave.

Could you please help us identify the issue.

[TDK]

> not able to read the signal using the differential ADC

Why not? What happens when you try? Can you show the code used to try to read the signal and the results you're seeing and what's wrong with them?

Note that the allowable input voltage range is 0 to VREF+ which means the signal must be biased at VREF+/2. If any portion of it is below 0, it will not work.

[Samudra]

Hi Sir,
We are applying a 1 V peak-to-peak signal to the amplifier input. Please find the simulation results attached for your reference.

The VOCM of the ADA4930 is connected to a 1.25 V supply, and the VREF of the ADC is 2.5 V.

Do we need to apply a DC bias at the amplifier input in this configuration to ensure proper ADC sampling? Kindly clarify.

[Samudra]

Hi Sir,

We are now able to read the sine wave using the ADC. The input is converted from single-ended to differential before being fed to the ADC. However, we are observing that the amplitude is limited to 500 mV.
We are applying a 1 V peak-to-peak input signal, but the ADC is only capturing 500 mV peak-to-peak at its input.

Could you please check this issue?
We have attached the ADC output and amplifier configuration for your reference.

Best regards,

Samudralankaiah.

[TDK]

With VREF+ = 2.5V, differential voltage is (VREF+) * 2 * (counts - 32768) / 65535. You are missing this factor of 2 in your calculations.