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    Unstable Output from 16bit ADC (ADS7825)

    Hi,
    I am trying to make a precise voltmeter(upto 1mV) using a 16bit ADC [ ADS7825 : http://www.ti.com/product/ads7825 ] and a pic micro. Which I will calibrate with an bench multimeter in my lab. I have interfaced the ADC in parallel mode. And I am getting the output almost as desired.. I have made the circuit on the breadboard, and it might be producing some noise(i am not sure about it). I am using channel zero as input and all other channels are grounded. Also since the circuit is built on one breadboard, the analog and digital grounds are also same.

    But the problem is that the output of the circuit is not stable. I am getting a variation of about 200mV !!!
    Below is the pic and video of my circuit showing output on LCD. 1st line on LCD shows the voltage measured and 2nd line shows the 16bit binary data received from the ADC. In video the analog input is connected to +5V supply. which have a large 470uF capacitor to ground.

    Click image for larger version. 

Name:	DSC_0015.jpg 
Views:	6 
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ID:	87548



    I don't know whats the problem, PLEASE HELP...

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    Re: Unstable Output from 16bit ADC (ADS7825)

    Where is circuit?

    Put capacitor on ADC input, try 100nF first then 1uF...
    “Let the future tell the truth, and evaluate each one according to his work and accomplishments. The present is theirs; the future, for which I have really worked, is mine.” Nikola Tesla



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    Re: Unstable Output from 16bit ADC (ADS7825)

    I have connected the ADC in simple parallel mode as given in it's data sheet....also shown below...

    Click image for larger version. 

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    also there is a big capacitor(470uF) at between the +5V power supply and ground( so as to remove noise in supply voltage)..and I have connected input of my ADC to that supply itself(while taking the video).

    Note: ADC can take input voltage from -10V to +10V.



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    Re: Unstable Output from 16bit ADC (ADS7825)

    It's very difficult to get low noise on a solder-less breadboard with a mixed-signal circuit, no ground plane, grounds tied together, and long overlapping interconnect wires. The basic rule is that digital signals always wants to join the analog. 200mV analog noise would not be unexpected with your layout.

    If you really need lower noise then you need to use a well laid-out PCB or soldered breadboard with separate analog and digital ground planes tied together at one point and isolated analog and digital supply busses. Read the "Layout" section on page 15 of the ADS7825 data sheet.
    Zapper
    Curmudgeon Elektroniker


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    Re: Unstable Output from 16bit ADC (ADS7825)

    Quote Originally Posted by nikhilsigma View Post
    Hi,
    I am trying to make a precise voltmeter(upto 1mV) using a 16bit ADC [ ADS7825 : http://www.ti.com/product/ads7825 ] and a pic micro. Which I will calibrate with an bench multimeter in my lab. I have interfaced the ADC in parallel mode. And I am getting the output almost as desired.. I have made the circuit on the breadboard, and it might be producing some noise(i am not sure about it). I am using channel zero as input and all other channels are grounded. Also since the circuit is built on one breadboard, the analog and digital grounds are also same.

    But the problem is that the output of the circuit is not stable. I am getting a variation of about 200mV !!!
    Below is the pic and video of my circuit showing output on LCD. 1st line on LCD shows the voltage measured and 2nd line shows the 16bit binary data received from the ADC. In video the analog input is connected to +5V supply. which have a large 470uF capacitor to ground.

    Click image for larger version. 

Name:	DSC_0015.jpg 
Views:	6 
Size:	1.20 MB 
ID:	87548



    I don't know whats the problem, PLEASE HELP...
    how did you solve your problem then?
    I am facing the same problem . i am using the ads7825 in serial mode... and the output varies by some 700mV which is not correct. i have made the circuit on a general purpose PCB board and connected it with a 8051 microcontroller kit.
    if you solved the problem, what arrangements did you make for analog and digital power and analog and digital grounds? did you follow the procedure and connections given on page 15 of the datasheet or did you do something else. others may also reply if they have any ideas or suggestions.
    thanks in advance.



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