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ADS5294 input filtering

Other Parts Discussed in Thread: ADS5294

Hi,

The datasheet for ADS5294 p.55 application information states the need for 5Ω - 15Ω resistor in series with each analogue input, prior to the differential input capacitor.

Our design has a 15pF 0402 C0G capacitor across the differential inputs, plus 12pF from each input to ground, with a 383R + 200R voltage divider providing the input signal at 10-40MSps. All these components are within 5mm of the ADC pins; we also have a video amplifier circuit connected by up to 2” of tracking. See attached picture for a schematic.

The application is for sampling video pixels, so each pixel value presented to the ADC will be settled prior to the sample being taken. (For the faster video rates, we'll adjust the component values to increase the bandwidth!) I'd appreciate your thoughts on whether this is a suitable input stage, or whether the series resistor is critical for a low noise design?

Thanks,

Tim.

  • Tim,

    We have received your post and hope to have a response back to you soon.

  • Hi Tim,

    We recommend to put a small valued series resistor at the ADC inputs when possible to act as a dampening resistor for the sampling 'kickback' glitch from the ADC.  The lower the impedance presented to the glitch, i.e. the lower the source impedance of the driving amplifier, the smaller the glitch will be.  If the impedance is sufficiently small then the series resistor may not be needed.  As an example, some of our ADC EVMs use the TPS770006 ampliifer to drive the ADC without this series resistor.  This amplifier has an output impedance of 4.4ohms at 100MHz.

    Regards,

    Christian

  • Thanks Christian. It's good to know there's some EVMs that don't use a series resistor at all. What do you think of having the much larger resistors acting as a resistor divider in our design?

    Tim.

  • Hi Tim,

    Unfortunately, we don't have characterization data for all possible interface circuits. We just know, the lower the impedance presented to the ADC, the smaller the glitch. If your ADC driving circuit has low output impedance, its impedance will dominate over the voltage divider and set the impedance seen looking out from the ADC. I think you will just have to try your circuit and confirm if the sampling glitch poses any issues in your system. Out of curiosity, what is your end product ?

    Regards,
    Christian