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 <forrestc@imach.com> wro=
te:
> I'm currently using an INA196 high side shunt amplifier in a couple of
> applications. =A0 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. =A0 I could d=
o
> 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. =A0 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. =A0 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. =A0 Some are close (the AD8207 for instance looks great until you
> realize the +65V is only at 5V). =A0 I'm ok with one which has an
> integrated ADC as well... =A0 =A0I 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. =A0 A
> depletion fet seems reasonable, except the high Ron which seems very
> likely to mess with the accuracy of the INA196. =A0Other options seem
> prohibitively expensive. =A0Add 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
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