AD7626 Ref input current value

Hi to all,

my acquisition and processing board has 12 AD7626 channels interfaced with a FPGA.

I have to provide a voltage reference source for all ADCs in simultaneous acquisition mode at 10MSPS.

So, following the application note "Voltage Reference Design for Precision Successive-Approximation ADCs" by Alan Walsh,

I choose the topology using a unit gain op-amp amplifier for each ADC (each op-amp fed from a unique ADR4540 voltage reference): see the topology below suggested in the AD7626 datasheet.

Supposing a AD7626 Vref input current value of 3.3mA average (10 times the AD7980; 1MSPS Pulsar ADC), I should

select a op-amp with internal output impedance low as 0.01 Ohm at 10MHz: this value is too low even for hi-speed op-amp like ADA4841.

So my question is regarding the AD7626 Ref input current value: is my hypothesis correct (Iref = 10 * Irref_AD7980)?

May be the AD7626 has a different internal topology and, for this reason, the Iref value is negligible and is not provided?

Thank you for any help.

reference_design.pdf
Parents
  • 0
    •  Analog Employees 
    on Nov 20, 2014 6:18 PM

    Hi Marco,

    I work for Analog and help support this product. The 600uA typical ref current comes from measurement data. I agree that this info should be added to the datasheet and will make a note to add it on the next revision.

    You are correct that the ref current will have strong harmonic content at the sampling rate (10MHz) and as i explained this is squashed by the reference reservoir capacitor, not the buffer. The buffer helps to top up the reservoir cap between conversions and also handle any lower frequency ripple due to modulation of the reference with the input signal. The buffer also handles the DC average current draw so the ref does not droop too much as discussed. If you were to plot the impedance of the buffer and cap versus frequency you would see the buffer output impedance increase with frequency and the capacitor impedance decrease with frequency. With both of them working together you hope to keep the impedance of the ref driver circuit low across the frequency range.

    Best Rgds,

    Alan

Reply
  • 0
    •  Analog Employees 
    on Nov 20, 2014 6:18 PM

    Hi Marco,

    I work for Analog and help support this product. The 600uA typical ref current comes from measurement data. I agree that this info should be added to the datasheet and will make a note to add it on the next revision.

    You are correct that the ref current will have strong harmonic content at the sampling rate (10MHz) and as i explained this is squashed by the reference reservoir capacitor, not the buffer. The buffer helps to top up the reservoir cap between conversions and also handle any lower frequency ripple due to modulation of the reference with the input signal. The buffer also handles the DC average current draw so the ref does not droop too much as discussed. If you were to plot the impedance of the buffer and cap versus frequency you would see the buffer output impedance increase with frequency and the capacitor impedance decrease with frequency. With both of them working together you hope to keep the impedance of the ref driver circuit low across the frequency range.

    Best Rgds,

    Alan

Children
No Data