How to offset or shift of voltage

[Circuitous]

Mostly, I stick to the digital side, so I haven't done a lot with op-amps, but...

I need to measure the AC output voltage of a transformer.  Not across the entire range, I’m really only interested when the voltage is between 6.8V and 11.4VAC, anything outside that will be a brownout or over-voltage situation.  So, to get good resolution on the ADC, instead of just using a voltage divider, I’d like to shift the input voltage down to be a 0-4.096V range, without compressing the range any more than necessary.

What methods do you recommend to offset the voltage like that?

[Neilm]

Dave did a video on peak hold circuits - probably about a month ago [Edit it is episode 490]. I would rectify the signal and use one of them with an op amp following the peak hold circuit to divide the signal down by 4.

Neil

[olsenn]

Google "voltage clamp"

[Circuitous]

I can implement a voltage divider, that's easy, but I loose the resolution.  I really want to shift that voltage down and take full advantage of the ADC's range.

@olsenn:  Hmm... "negative biased voltage clamp", looks like the right direction for me to head in.  I had tried something similar, but not quite.  I'll try some variations of that tonight.  Thanks.

[SebG]

Hi.  You could use a voltage divider and still get a full range ADC reading by using Vref- as the bottom reading value.  For example if you divide the input voltage by 2.8 and use a Vref+ = 4.096 and Vref- of 2.4V (which you can make using another voltage divider and maybe a capacitor or voltage follower) the full scale range will be 4.096V x 2.8 = 11.4688V  down to 2.4V x 2.8 = 6.72V (ie. 11.4688V - 6.72V)  This however makes the math a bit more complicated since (if using say 10 bit ADC) each bit's value will represent (11.4688 - 6.72) / 2^10 = 4.6375mV.  So if you get a ADC reading of say 128 the voltage measured will be (128 x 0.0046375 V) + 6.72 = 7.3136V. 

Don't forget to ensure the voltage at the input to the ADC is not less then ground when the AC voltage swings to it's negative side.  This could cause latch up or excessive current flowing.