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'[EE] High Side Shunt Amplifier'
2012\04\22@044946 by Forrest Christian

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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?

-forres

2012\04\22@104543 by Mark Hanchey

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On 4/22/2012 4:49 AM, Forrest Christian wrote:
> 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.

The closest thing I could find is the Max9611.  It runs from at least 2.7V and supports -.3v to 60V  and differential max of +-65V.   It is a bit more than just an amplifier, it includes ADC and i2C interfaces but the price doesn't seem too high, showing $2.68 at mouser qty 1.

Hope you find something that works.
Mark Hanchey


2012\04\22@115828 by Dwayne Reid

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At 02:49 AM 4/22/2012, Forrest Christian 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.

Can I suggest a somewhat different approach?

Allegro makes some fine hall-effect current sensors for probably less money than the INA196 op-amp that you are currently using.  The larger units are intended for high current operation (200A plus) but the tiny units are rated for plus minus 5A full-scale.  And a whole whack of current ratings in-between.

For example, the Allegro ACS712ELCTR-05B-T (Digikey # 620-1189-1-ND) sells for Can $4.84 in singles but drops down to just over Can $3 in hundreds.

The advantage of using a current sensor like this is that the current loop is galvanically isolated from the measurement side.  The ACS712 datasheet says that the current loop is about 1.2 milli-ohms and the device is rated at 2.1kV minimum isolation between the loop and the sensor.  Frequency response is stated at 80KHz.

There may be reasons why these parts aren't suitable but I think that they are worth a look.

dwayne


PS - I have another suggestion that I will post after this one.

dwayne

-- Dwayne Reid   <spam_OUTdwaynerTakeThisOuTspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing

2012\04\22@120432 by Dwayne Reid

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At 02:49 AM 4/22/2012, Forrest Christian 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've handled this in a couple of different fashions.

1) Just rely upon well-matched high-value series resistors to the op-amp and add clamp diodes across the op-amp inputs.

2) Use another (small) FET who's gate is tied to the gate of the main FET - use that small FET to disconnect the sense lead from the main FET while it is turned off.

I've used both techniques successfully in the past.

dwayne

-- Dwayne Reid   <.....dwaynerKILLspamspam@spam@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing

2012\04\22@163411 by Sean Breheny

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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 <forrestcspamKILLspamimach.com> wrote:
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>

2012\04\22@200125 by Forrest Christian

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Yeah, I had ran into the MAX9611/9612 myself.   It's one of those 'so close' type of options.   I actually really like the idea of the ADC integrated...  If it was even 5V more on the top end, I wouldn't be having this discussion...   It's just a bit too close for my comfort.  It might actually be where I end up back at.

Theres also a MAX9643 which is amplifier only which is sort of the similar situation.

-forrest

On 4/22/2012 8:44 AM, Mark Hanchey wrote:
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>

2012\04\22@200938 by Forrest Christian

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I agree, those allegro parts are intriguing.   I actually was looking at the ACS711 at some point... low cost, works, etc.

I guess where I'm hung up is that I really don't want to use a different solution where I'm measuring current on-board vs a customer provided shunt.   I might end up there, just because the solutions aren't all that great for the onboard option as-is.

-forrest

On 4/22/2012 9:58 AM, Dwayne Reid wrote:
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>

2012\04\22@201211 by Forrest Christian

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I'm considering #1, although I don't think the INA196 is the correct solution for this as there are warnings in the datasheet mainly because it's designed for a low-impedance connection to the shunt...  I am considering rolling my own with some precision matched resistor pairs..

On #2... Yep, close to being there myself.   I was hoping not to end up there, although it's looking more and more likely.

-forrest

On 4/22/2012 10:04 AM, Dwayne Reid wrote:
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>

2012\04\22@201404 by Forrest Christian

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We're literally taking customer provided shunts so it's hard to answer.  Generally these are 100mV shunts, but may be anywhere between 1A@100mV to 40A@100mV or more.

In regards to the shorted across... mainly I was saying that a traditional clamp diode or similar (without series reisstors) won't work.  In addition, the INA196 doesn't like series resistors at all..

-forrest

On 4/22/2012 2:34 PM, Sean Breheny wrote:
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2012\04\22@220230 by Neil

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Haven't read all of this thread yet, but have you checked Zetex parts?  IIRC ZXCT1082 or 1084 goes up to 60V, and can be powered from 3.3V.  They're not integrated with ADC etc, but very low cost (under $1 in quantity IIRC).

Cheers,
-Neil



On 4/22/2012 8:01 PM, Forrest Christian wrote:
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