> How much current are you trying to measure with this (i.e. what is the
> max current? Also, what is the smallest current you want to measure
> accurately?)
>
> When you say that you want to protect the sense amplifier when the FET
> is off and that you cannot do that by shorting around the FET, do you
> mean to imply that you cannot tolerate even a tiny current flowing
> through your protection circuit when the FET is off (like, for
> example, 10s of microamps)?
>
> Sean
>
>
> On Sun, Apr 22, 2012 at 4:49 AM, Forrest Christian<
.....forrestcKILLspam
.....imach.com> wrote:
>> I'm currently using an INA196 high side shunt amplifier in a couple of
>> applications. As I asked/discussed a couple of weeks ago, one of them
>> involves current monitoring the currents through some P-channel FETS,
>> and am looking at moving the sensing from a sense resistor to sensing
>> across the FETS.
>>
>> I of course need to deal with the situation where the FET is off,
>> producing a high differential voltage across the amplifier. I could do
>> this easily in software EXCEPT the INA196 only handles +-18V of
>> differential input, and this happens to need to support up to around 62
>> or 63V... which is fine with the common mode range on the INA196, but
>> like I said not so much on the differential spec.
>>
>> I also use the INA196'es on a couple of other circuits to monitor
>> customer-provided shunts, and have seen some failures related to what I
>> believe is excessive differential voltage inputs as well. For this
>> reason, I'd like to avoid fixing the above problem with something driven
>> along with the FET, and instead handle this as a separate circuit.
>>
>> I think at this point, I need to either protect the INA196 from a high
>> differential input, or switch to a difference amplifier. However, I'm
>> finding this to be a struggle for the following reasons:
>>
>> 1) I am unable to find any difference amplifier which runs at 3.3V and
>> supports the necessary common mode range of 0 (-2 would be better) to
>> 65V. Some are close (the AD8207 for instance looks great until you
>> realize the +65V is only at 5V). I'm ok with one which has an
>> integrated ADC as well... I also am hoping for something which is a
>> $1-$2 part, not a $5 part, since I use a lot of these on these boards.
>>
>> 2) Adding protection to the INA196 seems doable, except it also needs to
>> protect 'open' (I can't have it shorting across the shunt and taking
>> current), and have minimal impact on the accuracy of the INA196. A
>> depletion fet seems reasonable, except the high Ron which seems very
>> likely to mess with the accuracy of the INA196. Other options seem
>> prohibitively expensive. Add the fact that we're turning the fets off
>> once a second for a few microseconds and this circuit has to be
>> transient free... well>18V transient free I guess is more accurate.
>>
>> I'm hoping someone knows of a good, relatively inexpensive, difference
>> amp which I could use in this circuit, OR alternatively has an idea for
>> a relatively inexpensive protection circuit which effectively
>> disconnects the INA196 when a differential voltage over a certain
>> threshold is seen.
>>
>> Anyone have any ideas?
>>
>> -forrest
