Its probably easier/cheaper to construct a suitable filter for a GSPS ADC than
to construct a TAC that is fast enough to suit an ADC with a GHz clock.
Minimising the emitter to emitter inductance of a longtailed pair or equivalent
is key to achieving a fast enough switching time for a suitable TAC. The reset
switch also needs to have sub nanosecond turn on and turn off. Saturated
bipolar switches are too slow. The classical dual diode clamped reset switch
driven by a fast switching current source should work well if the parasitic
interconnect inductances can be kept low enough.A custom IC is probably the
only effective solution for such a TAC. The somewhat heroic measures employed
in the Wavecrest counters is perhaps the limit of discrete construction
techniques.
Bruce
On Tuesday, 10 May 2016 1:08 AM, Bruce Griffiths
<[email protected]> wrote:
Since the GSPS sampling ADCs all appear to use an input buffer with relatively
low value resistors between the differential inputs or connected to a midpoint
bias voltage, some kind of high impedance buffer is needed between the TAC
capacitor and the ADC input when using such ADCs. The highest clock frequency
for capacitive input pipeline ADCs appears to be about 300MHz. A 1GSPS ADC
relaxes the performance requirements of the TAC significantly making it easier
to achieve 1ps or less noise.The trade off is that a discrete TAC may not be
fast enough due to parasitic bond wire inductances etc. All monolithic TACs
with 1ps resolution have been designed using standard IC processing.
Bruce
On Tuesday, 10 May 2016 12:14 AM, David <[email protected]> wrote:
Sure, and then we are back to a transition midpoint timing TDC. Or AC
couple it for a centroid timing TDC. These require a lot more
processing to generate a result compared to a time to amplitude
converter but with economical FPGAs and ARM microcontrollers, maybe
this does not matter.
I was just wondering about the speed limitations of a time to
amplitude based TDC. I am more comfortable with analog design than
using FPGAs.
On Mon, 9 May 2016 03:35:46 +0000 (UTC), you wrote:
>Another option is to use a low pass filter to increase the transition times of
>the signal to be timestamped and use a pipelined ADC to sample the filter
>output.Perhaps something like the attached filter derived from:
>http://bears.ucsb.edu/rad/pubs/conference/MTT_S_2004.pdf
>May be effective in that it has near Gaussian response with relatively low out
>of band SWR.
>
>Bruce
>
>On Monday, 9 May 2016 3:01 PM, David <[email protected]> wrote:
>
>
> How much will dielectric absorption in the capacitor affect the
>accuracy of the result with such a high conversion rate? I am used to
>dealing with it on much longer time scales and higher resolutions.
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