[time-nuts] ACAM GP22 Chip

Thomas Allgeier th.allgeier at gmail.com
Tue Nov 24 09:04:32 EST 2015


I have an ACAM GP22 TDC chip and evaluation board which I am looking at for “work” purposes – I work for a company active in the weighing and force measurement world.

I should say from the start that I am new to time and frequency measurements and not even an electronics engineer – but then I have been exposed to high-precision electronics for the last 25 years hence have picked up some dangerous degree of half-knowledge.

We want to use this chip to measure the period of a square wave, of around 13 kHz i.e. in the 70 µs range. As the application is potentially high-accuracy we need to know the period to within 1 ns or better.

In order to evaluate the chip I was planning to replicate John A’s experiment with the coaxial delay line from the HP5370b – but as my interest is in “measuring range 2” of the GP22 I need a delay of 500 ns or more (actually 1 µs sounds a better start). This is the equivalent of a 200 m length of cable. I fear trouble with this: Am I not getting unwanted inductivities if I use a coil of that size?

So, to come to the point: Am I pushing the concept of a coax delay too far with 1 µs and are there other (simple/reliable) ways to achieve this kind of delay? I have tried it with a shorter piece of cable (around 2 ns which is measured in “range 1”), there I seem to get consistency virtually to within 100 ps. But I need to know if the device sticks to this level of performance when the periods are much longer, and thus measured in “range 2”.

Thanks and best regards,


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