We're literally taking customer provided shunts so it's hard to answer. =20 Generally these are 100mV shunts, but may be anywhere between 1A@100mV=20 to 40A@100mV or more. In regards to the shorted across... mainly I was saying that a=20 traditional clamp diode or similar (without series reisstors) won't=20 work. In addition, the INA196 doesn't like series resistors at all.. -forrest On 4/22/2012 2:34 PM, Sean Breheny wrote: > 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 w= rote: >> 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 >> -- >> http://www.piclist.com PIC/SX FAQ& list archive >> View/change your membership options at >> http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .