[time-nuts] Some info on the ACAM TDC and a question to Brooks Shera's PI controller

Achim Vollhardt avollhar at physik.uzh.ch
Wed Oct 20 13:58:12 UTC 2010


Dear Group,

it might be of interest, that I have some measured data on the ACAM
TDC-GP2 chip, which I am using for another project in our lab here.
Single shot resolution of 54 ps rms was quickly achieved, which is close
enough to the 50 ps quoted in the datasheet for me to believe it :)

As I am willing to use this device in a GPS-locking device, I have of
course set my eye on the PI controller from Brooks Shera. He is quoting
using a second order IIR filter, but as a newbie in digital filters, I
have problems to write suitable code for it..

I did find however an easy implementation of an PI controller:

http://www.atlantixeng.com/pdf-files/PID_Controller_in_FPGA.pdf

I(k) = (E(k)+E(k-1))*Ki*Ts/2 + I(k-1)
P(k) + Kp*E(k)

P(k) and I(k) being the proportional and integral outputs at sample 'k',
E(k) being the error input. Ts is the sampling rate, Kp and Ki are the
gain factors for the two controller parts, which results in a time
constant being Tn=Kp/Ki.

Is this a suitable approach for a control loop (assuming proper choice
of gains)?

Is an IIR filter better suited? As far as I can tell, this is not IIR..

Thank you,
Achim, DH2VA




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