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ADS1675REF: Unbalanced input of the differential amplifier on the ADS1675REF board

Part Number: ADS1675REF


I would like to use the ADS1675REF board to sample a single-ended signal, but before I started,  I found if J1 and J3 are floating, the ADC measured voltage is about 0.3V, J3 has 1.0V potential and J1 has 0.7V potential, while if J1 and J3 are short-ended, the ADC measured voltage is about -1.7V. I shorted J1 and connected J3 to a DC power supply, the ADC measured voltage is never equal to the DC power supply's output and shows an abnormal relationship. 

Can anyone explain why? how can I use the ADS1675REF board to sample a single-ended signal? Is this problem related to the wrong use of the differential amplifier THS4503 as shown in the attached figure? Kindly note I removed the R44 which I thought was useless and might cause problems.

Many thanks in advance.

  • Hello,

    It seems like the power supplies are not correct.  Please confirm that you have correct power to the board.

    The minimum supplies needed for the default configuration (R39 installed, JP2 and JP4 installed):

    There is also an additional jumper for the THS4503 negative supply, JP1, that needs installed from pin 2-3.

    If everything is powered correctly, with J1 and J3 floating, you should measure 2.5V, which is set by the REF5025, U1.

    Keith Nicholas
    Precision ADC Applications

  • Thanks a lot!

    I applied +9V and -4V to +Vcc and -Vcc of the differential amplifier THS4503, the board operated better: I can get -Vin if I applied Vin to J1 using a DC power supply with J3 short-ended. But I still get wrong measured value if I applied Vin to J3 and short-ended J1. 

    By the way, it says in the datasheet of THS4503, the differential amplifier can be powered with 5V, and the common voltage is 2.5V in this use, so I applied +5V and GND to its +Vcc and -Vcc to reduce the need for different power sources, can you explain why this configuration failed to work properly? Thanks!

  • And I can detect 2.5V at Pin2 but I cannot detect this common voltage at each of the output Pin5 and Pin4... I don't know why..kind of lost..


  • Hello,

    What is the range of the single-ended input signal that you want to measure?  Does it range from -2.5V to +2.5V, or does it range from 0V to 5V, or some other span?

    The input voltage range on the THS4503 is more limited when using a single 5V supply.  The voltage on either pin 1 or pin 8 is limited from 1V to 4V typical.  When using a 0->5V single ended input voltage, the input common mode voltage spec of the THS4503 will be met.  However, if your input range is from -2.5V to +2.5V, then the input common mode spec will be violated with the input at -2.5V.

    Please connect J1 and J3 to ground (0V) and then take the following measurements to confirm the THS4503 is working correctly:

    All measurements with respect to GROUND (pin 4 of U1)

    1. Measure pins 1 and 8, should be about 1.25V each.

    2. Measure pin 2, should be 2.5V, set by the REF5025 (U1).

    3. Measure pins 4 and 5, should be about 2.5V each.

    4. Measure pin 3 (+supply, +9V) and pin 4 (-supply, -4V)


  • Thanks a lot!

    I learned a lot. The signal we want to measure was a small signal which was found below +-1V, so I still have to use a negative power supply. 

    And I did a test on THS4503 as you suggested, the results are as follows:

    *Condition: J1 and J3 are shorted-ended using shorting caps, R44 is removed, +9V and -4V powered.

    *Results: P1: -0.336V, P8:-0.241V, P2: 2.5V, P3: 9V, P6: -4V, P4: -0.673V, P5: -0.484V.

    And I did more tests and found that the THS4503 now is unable to output positive voltage, but can still function when the output voltage is expected to be negative like when J1 is short-ended and negative voltage is applied to J3.

    Based on the test results, I am thinking the THS4503 is damaged and looking forward to your opinions. 

  • Hello,

    Yes, it is very likely the THS4503 is damaged.  There is a possibility that the ADS1675 is damaged, and loading the output pins of the THS4503.

    One additional test can be done:

    Remove R17 and R18 to isolate the THS4503 outputs from the ADS1675 inputs and retake the above measurements.  If you see similar voltages, then the THS4503 will need replaced.

    You can order additional THS4503 samples from the TI Store or from a TI authorized distributor.


  • Hi Keith,

    Thanks a lot. I tested the THS4503 after removing R17 and R18 as you suggested. The results are as follows:

    *Conditions: J1 and J3 are short-ended using shorting caps, power supply: +9.12V, -4.07V

    *Results: Pin1: -1.564V, Pin2: 2.5V, Pin3: 9.12V, Pin4: -3.12V, Pin5: -2.92V, Pin6: -4.07V, Pin7: -0.148V, Pin8: -1.464V.

    So now it should be pretty sure that THS4503 is damaged right? I have ordered several new chips and will be replacing the damaged one by myself. 

    Wish the ADC chip is all fine, fingers crossed... 

  • Hello,

    Yes, the THS4503 is almost certainly damaged.  The THS4503 outputs have stressed the ADC inputs, but there is a good chance the ADC will still function correctly.


  • Hi Keith,

    I replaced the THS4503 and the board now works all properly. Many thanks for your help!



  • Hi Xiaoyi,

    You are welcome.