ADC front-end common-mode bias may be missing after AD8331 AC coupling

[xquan-xj]

Summary

The ADC front-end in hardware/adc/PICO_ADC_Test appears to AC-couple the AD8331 VGA outputs into the ADC10065 inputs, but the ADC input side does not seem to have a defined common-mode bias.

This may leave VIN+ and VIN- of the ADC10065 floating at DC, instead of being biased around the ADC's required input common-mode voltage.

Relevant circuit path

From the KiCad netlist:

• AD8331 VOH -> FB1/R4 -> C2 -> R11 -> ADC10065 VIN+
• AD8331 VOL -> FB3/R8 -> C9 -> R10 -> ADC10065 VIN-
• C20 = 50 pF is placed differentially across VIN+ / VIN-
• ADC10065 VCOM only appears to be decoupled with C13 = 100 nF
• AD8331 VCM only appears to be decoupled with C19 = 100 nF

So the AD8331 output common-mode is blocked by the series capacitors C2/C9, but the ADC side does not appear to be re-biased to VCOM.

Datasheet concern

The AD8331 output defaults to approximately 2.5 V common-mode if VCM is not externally biased. The datasheet also notes that for DC coupling into a 3 V ADC, the AD8331 VCM pin can be biased to 1.5 V.

The ADC10065 provides a VCOM output around 1.45 V for setting the ADC input common-mode voltage.

Potential issue

Because C2/C9 AC-couple the signal, the original AD8331 2.5 V common-mode should not directly reach the ADC. However, the ADC input side still needs a defined DC operating point.

Without bias resistors or another bias network from ADC10065 VCOM to VIN+ and VIN-, the ADC input common-mode may be undefined, causing unreliable sampling, distortion, or startup-dependent behavior.

Suggested fix

Add a proper ADC-side common-mode bias network, for example:

• Bias both VIN+ and VIN- to ADC10065 VCOM through matched resistors near the ADC input.
• Choose the resistor values together with C2/C9 to set the desired high-pass cutoff.
• Keep R10/R11 and C20 as the ADC-side kickback / anti-aliasing filter if the bandwidth target is still appropriate.

Alternatively, if DC coupling is desired, bias AD8331 VCM to approximately 1.5 V and verify the RCLMP value per the AD8331 datasheet recommendation.