Driving multiple AD9236 from an External Voltage Reference

Hello,

The AD9236 is a rather vintage product released back in 2003.  Not sure why this product was selected but perhaps using the AD9637 Octal-80 MSPS ADC would be a better choice since all ADC's (i.e. 6 in your application) share the same voltage reference albeit each ADC has its own differential reference buffer.

The differential cap arrangement shown in figure 33 below was designed to provide a low impedance differential source for this ADC core.   More recent ADC's like the AD9637 include the differential ADC reference buffer on chip hence no need to for external capacitors. 

If gain and perhaps offset matching between ADC's is critical.......perhaps include a calibration circuit with the calibration circuit providing a fixed DC voltage representing ADC full-scale as well as AGND so that all ADC's can be calibrated to a single source.  Note this will require a switch in each ADC signal path and ideally one would place calibration circuit prior to any signal conditioning circuitry leading up to the ADC so that gain and offset errors from these sources can be also calibrated out.


AD9637 Datasheet and Product Info | Analog Devices

Hello,

The AD9236 is a rather vintage product released back in 2003.  Not sure why this product was selected but perhaps using the AD9637 Octal-80 MSPS ADC would be a better choice since all ADC's (i.e. 6 in your application) share the same voltage reference albeit each ADC has its own differential reference buffer.

The differential cap arrangement shown in figure 33 below was designed to provide a low impedance differential source for this ADC core.   More recent ADC's like the AD9637 include the differential ADC reference buffer on chip hence no need to for external capacitors. 

If gain and perhaps offset matching between ADC's is critical.......perhaps include a calibration circuit with the calibration circuit providing a fixed DC voltage representing ADC full-scale as well as AGND so that all ADC's can be calibrated to a single source.  Note this will require a switch in each ADC signal path and ideally one would place calibration circuit prior to any signal conditioning circuitry leading up to the ADC so that gain and offset errors from these sources can be also calibrated out.


AD9637 Datasheet and Product Info | Analog Devices

Hi PMH2023,

I've got this ADC as requirement based on previous projects.

Now I'm expanding number of analog channels.

When asking about capacitance I was referring to VREF pin external capacitors, and not to REFB and REFT.

Regarding AD9637, it's interesting part. Need to see how redesign fits to project timeline.

Hello,

Sorry about confusion.   No capacitors are required on the VREF pin when using external op amp based voltage reference  with the load presented by VREF pin being 7 K per pin  (i.e. divide 7 Kohm by  number of ADC's used to find overall loading).

I still have concern about frequency stability of External VREF buffer amplifier because I'm expecting impulse currents to be sinking into VREF pin.
May I consider VREF as DC type of load? 

Hello,

The capacitors tied to REFT and REFB are used to absorb the impulse currents from the ADC core hence VREF node can be considered DC load.  Note that internal reference (which you will disable) requires external capacitors for its own stability purpose (i.e. sets dominant pole).