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Negative and Positive Voltage Measurement with ADC

Other Parts Discussed in Thread: ADC78H90, LMP2021

Hi,

I want to voltage measure with ADC78H90. Voltage range: -30V - +30V. How do I measure these voltages with ADC78H90. 

Regards.

  • Hi there,

    ADC78H90 has an input range of 0 to 5V (with 5V supply). To accomplish your task you will need to attenuate the -30V to 30V input ranhge by a factor of 12, and level shift it to to eliminate the negative part of the range.

    After that conditioning is done, you can feed the resulting signal into the ADC input.

    Regards,

    tom

  •  I already know the shifting is required but my question is about how can I establish. What is the circuit for this operation. Could you please prefer a circuit schematic.

  • Hi there,

    It is difficult to suggest a solution when the constraints are not known. But here is a suggestion. The circuit is based on the simple divider and a unity gain buffer. The divider scales down the input, while the 2.5V offset source assures that the input signal to the buffer is always positive. LMP2021 has very low offset and offset drift. Its input bias currents are low enough so that the divider impedance will not affect your accuracy.

  • Hi tom,

    What's the reason that you use the LMP2021 ?  Is it convenient that we can apply only the voltage divider resistors output to the ADC channel. And when the ADC belongs to F28027 MCU instead of ADC78H90, is there any circuit design difference? Or is there any input impedance difference with ADC78H90 ADC input channels and F28027 ADC input channels ?

  • Hi Ozgur,

    You can certainly try feeding the divider center tap into the ADC. The issue that you may encounter is the trasient at the beginniong of TRACK period may not settle out fast enough - the transient is due to the switching of the sampling switch, and charge/discharge of the sampling cap indside the ADC. The transient will always be there, but you can shorten its settling by reducing you source signal impedance, hence the unity gain op-amp.

    I do not know the details of the F28027 MCU ADC, but my guess is that it is also an unbuffered SAR ADC, and therefore the sampling transient will also be there.

    Now, the divider in the sample schematic above is 11k and 750, so the equivalent source impedance is less than 750 which is not too bad. It may be woth while for you to just try feeding the divider output direcltly into the ADC - keep the 1n cap at he input of the ADC, though.

    Regards,

    tom 

  • Hi;

    Please note that circuit that Tom advices, is also a "differential amplifier" with unity gain. With this perpective, using a negaitve supply opamp, you may get more features out of it.

    In case of Tom's advice,I may also advice adding a bidirectional TVS  at divider output(opamp input) to make sure opamp inputs never exceed +- Vs.

      

    rha 

  • Can I use the same circuit to measure voltage at a load that sinks 2 amps?