AnsweredAssumed Answered

Conversion error for AD7657

Question asked by Kaos on Jan 27, 2014
Latest reply on Feb 20, 2014 by Kaos

Dear Sir/Madam,


As for your ADC AD7657, why the miss conversion is happen in sometimes(not every time)?

Do you have the similar case for this miss conversion in the past?


We are just proceeding to verify for convert error why it is sometimes happen.

The way of verify is doing conversion may time continuous (we say this "roop back") .

So we found the 1 miss conversion happen after 100 times conversion as following. But  some samples are not happen on 1M to 10M times.

Almost the data converted were no problem they were correct data.


Our confirmed

1)   1)  Voltage of Vref pin==Stable seems no problem

2)   2) Range pin==fixed to GND this means 4*Vref seems no problem

3)   3) Component layout external cap. of 3-REFCAPs==Put on the close IC pin seems no problem

4)   4) We just use the parallel I/F, use all 6ch.

5)   5) We caught the Unusual result as following chart.

   This means the reason is digital setting not analog reason.

6)   6)  We confirmed this trouble in our thermostatic oven 0C to 40C also, but it is brought the same result.

7)   7)  We found a couple of AD7657s for this miss conversion. Not only 1 AD7657.

8)8) What should we confirm our condition? Please give us your good advise.

8)     Thanks Kaos


input voltageCalculated valueTrouble data
9V7372.81CCC76801E009.38 ------
7.5V61441800511913FF6.25 ------
5V40961000204707FF2.50 ------
4V3276.80CCC35840E004.38 ------
2.5V20480800102303FF1.25 30720C003.75 409610005.00
1.25V102303FF409610005.00 ------
0V00000409610005.00 -40972FFF-5.00    
-1.25V-10243C00000000.00 ------
-2.5V-20483800-10243C00-1.25 000000.00 -40972FFF-5.00
-5V-40963000-20483800-2.50 ------
-7.5V-61442800-51202C00-6.25 000000.00    
-9V-7372.82333-768121FF-9.38 ------
-10V-81922000-800020C0-9.77 ------
1) The lower 8 bit of trouble data seems to be 00 or FF.
2) The upper 8 bit of trouble data seems to be shifted 1bit as for calculated value.
3) When the input voltage is 10V or -10V, we cannot confirm trouble conversion data.