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Offset measurement of latched comparators.

During comparator design one is often (but not always) interested in the systematic and mismatch-related offset. When dealing with continuous-time open-loop comparators the offset measurement methodology is similar conventional OTA offset measurement e.g. DC sweep, while latched comparators require a somewhat more discrete-like manner of measuring offset.

One possible approach is to use a very precise (long sloped) linear ramp, and using massive oversampling and comparator decision storage. This approach seems to be a bit tedious as its precision is solely dependent on the ramp's slope.

A basic illustration of the linear ramp method.
A basic illustration of the linear ramp method.

An alternative method for measuring offset of latched comparators would be the binary search method. One can possibly use an ideal DAC to apply "search" voltages and depending on the comparator's decision reach a settling value with a constant binary-weighted precision. E.g. to achieve 16 bits of search accuracy only 16 cycles would be required. Here are some basic illustrative examples of the two methods.

An illustration of the binary search method.
An illustration of the binary search method.
The binary search method for measurement can be applied in various ways. One practical implementation was to implement the algorithm using Verilog-A and directly add a "search" instance where you can test your comparator directly in your existing SPICE simulation. Here is a very simple example of a possible implementation.

// VerilogA for daisyCycAd, daisyCycAdBinSchSAR, veriloga
// A component handy in comparator offset measurement. Uses a binary search algorithm with a "dive" coefficient of 2. See comments for more information.
// Initial version P1A - Deyan Dimitrov

`include "constants.vams"
`include "disciplines.vams"

module daisyCycAdBinSchSAR(vCompIn, vCompOut, vClk, vdd, gnd, vCompRef);

input vCompIn, vClk;

output vCompOut, vCompRef;

inout vdd, gnd;

electrical vCompIn, vCompOut, vClk, vdd, gnd, vCompRef;

parameter real vTransClk = 1.65;
parameter real vTransComp = 1.65;
parameter real vSchTop = 3.3;
parameter real vSchBot = 0;
parameter real vCompReference = 1.5;
//parameter integer Resolution = 16;
parameter integer NrOfCodes = 65535;
//parameter real tCompSpeed = 100e-12;
real vRefInReal;
real vBinSch;
real vComp;
real scharray[0:NrOfCodes];
//real next;
integer imid;
integer imin;
integer imax;
integer i;
integer codes;

analog begin


       vBinSch = vSchBot + (vSchTop-vSchBot)/2;  // Mid point as a start of the search

       imin = 0;
       imax = NrOfCodes-2;
       imid = (NrOfCodes-2)/2;

       	   for (i = 0; i < NrOfCodes-1; i = i + 1) begin
       	     scharray[i] = (((vSchTop-vSchBot)/NrOfCodes)*i)+vSchBot;   // Creating reference array

	   $strobe("Array Max %g", scharray[imax]);
	   $strobe("Array Min %g", scharray[imin]);


       @(cross(V(vClk) - vTransClk, 1)) begin

//       next = $abstime + tCompSpeed;    // Possible internal compensation for comparator's delay
//       end
//       @(timer(next)) begin
//       $strobe("Imax: %d", imax);
//       $strobe("Imid: %d", imid);
//       $strobe("Imin: %d", imin);

       vComp = V(vCompIn);   // Strobe comparator decision
//       vRefInReal = V(vRefIn, gnd);

       if (imax >= imin) begin	// Continue searching if imax >= imin

       	  if (V(vCompIn) > vTransComp) begin      // Find-out which sub-array to search
//       	  vBinSch = vBinSch + (vSchTop-vBinSch)/2;
		  imin = imid + 1;	          // Change min index for the upper sub-array
		  imid = imin + ((imax-imin)/2);  // Update imid to be used for strobing-out
//       	  vBinSch = vBinSch - (vBinSch-vSchBot)/2;
		  imax = imid - 1;		  // Change max index for the upper sub-array
		  imid = imin + ((imax-imin)/2);  // Update imid to be used for strobing-out


       vBinSch = scharray[imid];		  // Look-up at the reference array and assign to vBinSch (Votage to be strobed-out)

       V(vCompOut) <+ vBinSch;			  // Update search voltage
       V(vCompRef) <+ vCompReference; 		  // Update comparator reference



The whole principle is simple and self-explanatory. Some delay between the latch clock and S/H clock of the search component is required to compensate for the comparator's speed. Here are some practical usage illustrations.
A primitive testbench proposal.
A primitive testbench proposal.
Component settings.
Component settings.
Offset search progress.
Offset search progress.

If you find this useful the component's symbol for Virtuoso 6 as well as the *.va code can be found here.

Date:Sun Mar 11 12:46:50 CEST 2014


08 Jul 2017, 02:19

Thank you very much for sharing such a fantastic experience.

Would you please share ADE too? or give more details on the parameters? I want to resimulate the test bench with Verilog-A comparator and check with the result you showed to make sure that I am in aright path.

Many Thanks
Deyan Levski
10 Jul 2017, 13:28
Hi Emi,

This block basically forms an ideal successive approximation DAC with one sample delay. Here's a brief info on the parameters and their use:

vTransClk - threshold for the clcok
vTransComp - threshold for the comparator transition
vSchTop - high output voltage level
vSchBot - low output voltage level
vCompReference - reference voltage for the comparator
NrOfCodes - number of sar search codes this determines the iterations

I think you can try pasting this veriloga module into an indivisual block and play around with the testbench. I am not sure if I keep all ADE simulation states for this module, but it really is straight forward and nothing complicated.

Thanks and let me know if you haven't wrapped it around.
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