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ADC1175 design questions

Eric Eckenrode
Intellectual 275 points
Other Parts Discussed in Thread: ADC1175, LMV796, LMH6702

Hi,

The ADC1175 has a dynamic capacitive load (4pF when the clock is low and 11pF when the clock is high).  Is this why there is a 62pF capacitor in the reference circuit on page 16 on the datasheet?  Can I get away without a capacitor there?


I only need a GBW of about 10MHz.  I am having trouble determining a general purpose OpAmp that can adequately drive the dynamic capacitance.   Can I use general purpose OpAmp like the LMV796?


On the bottom reference (VRB) the datasheet shows a 1N4148 diode.  I would assume I should remove this since I will be setting VRB to 1.8V


Also, when generating reference voltages (pg. 18), it is suggested that 10uF tantalum caps are used.   What are the drawbacks of using ceramic or electrolytic?


Thanks,
Eric

 

  • 0
    TI__Expert 4340 points

    Eric,

    The 62pF cap in the pg.16 schematic limits the BW to provide an anti-aliasing filter, reduces the swing of capacitance (as a %) between the two phases, and provides a sink for charge kickback during switching transients. If you do not have a capacitor there, then the following can happen:

    - increased noise due to aliasing of a wide input noise BW

    - decreased dynamic performance due to insufficient supression of charge kick-back transient

    You need some way of suppressing the kick back charge transient. You can do that with a larger cap well, or you can try to settle it fast with a driving amp. If trying to suppress with a driving amp, you have to minimize the series resistance between the amp and the ADC which is not good for the stability of some amps (as discussed on pg.11 of LMH6702 datasheet).

    I don't think that the dynamic capacitance is the problem, rather it is important that the amplifier can settle fast enough to resolve the charge switching transients within the sampling period. The amplifiers listed on pg.15 of the ADC1175 are good choices.

    As for the 1N4148, this is used to clamp the voltage of the lower reference during switching transients where charge is push/pulled out of the VRB pin. The VRB driver operates on a -5V supply so the VRB pin node could potentially drop to a large negative voltage during a transient. The diode prevents that.

    At the time the d/s was released, I don't think 10uF ceramic caps in small sizes were available. 10uF ceramics with qualities superior to Tant and Elect. caps are now widely available. At this point, I think Tant caps are only somewhat competitive in >100uF spots where lower ESR and small size is needed. Electrolytic/Aluminum gets you the best cost for >100uF.

    Regards, Josh