> On 23/06/2011 15:10, Forrest Christian wrote:
>> I have a circuit which I now plan to use an allegro ACS711 current
>> sensor.  This puts out voltage based on current, where zero current is
>> referenced to VCC/2.   As I only care about current in one direction,
>> I'd really like to remove this offset and then be able to amplify this
>> to a range based on my expected current range.
>>
>> Vcc is 3.3V.  The  full scale output is from 1.65 to 3.3V.  The current
>> range I am working with is probably more like 1.65V to 2.5V. I'd like to
>> move this to where this is scaled to a range of around 0 to 3.3V.
>>
>> The obvious way to do this is to feed this output into a difference
>> amplifier made out of an opamp and four resistors.  Wire the positive
>> input to the ACS711, and wire the negative input from a VCC/2 voltage
>> source made from another opamp in unity gain mode and with a voltage
>> divider on the input.  But that seems like a bit of overkill since I'm
>> pretty certain I should be able to do this all with one opamp and skip
>> the unity gain amp- unfortunately I'm not seeming to have much luck
>> doing the correct analysis to make this work.
>>
>> So, I'd appreciate any opamp wizards which could either just point me
>> towards the solution, or even better help me figure out where my 'mental
>> model' of opamp operations is falling down in this case, so I don't have
>> to ask a similar question again.
>>
>> Thanks,
>>
>> -forrest
> what is resolution of ADC compared to accuracy of the sensor? Does it
> actually matter if full adc range isn't used? If so save on HW and just
> scale in SW. Unless you use precision resistors etc adding an op-amp may
> reduce accuracy?
>

I agree with this! Adding a differential amplifier in front of the ADC in
this case looks like you're only gaining 1 bit of resolution (converting a
half scale range to a full scale range). You'll probably lose that in
errors in the diff-amp.

I did a "cow thermometer" design years ago (see http://www.gla-ag.com/ )
that uses a precision thermistor. A typical thermistor circuit puts the
thermistor in a Wheatstone bridge with fixed resistors in the other legs.
This results in one side just being a voltage divider feeding one side of
the differential amplifier, getting rid of offset from the voltage divider
that has the thermistor in it. So, you'd need three precision resistors.
Instead, I increased the resolution of the ADC and then used one precision
resistor to pull up the thermistor to the ADC reference voltage. No
differential amplifier to contribute errors. Only one precision resistor
required. No calibration required. It just works.

Harold
(The ideal design has zero parts).



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