INA168/138 input current

[Guest]

Hi,

I'm planning to use the TI INA138 current-chunt monitor to measure the
load current out of a 6-cell NiMh pack. The INA138 will be permanently
connected.

I'm keen to keep the standby current from the battery to a minimum when
the product is powered off; the datasheet says the worst-case current
into either of the sense pins is 10uA (which is fine). What it doesn't
say is whether this still holds with the Vcc pin tied to GND - the 5V
rail powering this pin also powers a bunch of other stuff, which will
have a fairly-low resistance path to GND when it's off.

I've been bitten like this before with protection diodes etc which
aren't shown on the equivalent circuit, anybody have any experience with
this part? The fall-back plan is to isolate the 138 inputs with a pair
of PMOS's, but it'd be a lot neater if this wasn't necessary.

TIA

R.

 

[Jim Thompson]

Hi,

I'm planning to use the TI INA138 current-chunt monitor to measure the
load current out of a 6-cell NiMh pack. The INA138 will be permanently
connected.

I'm keen to keep the standby current from the battery to a minimum when
the product is powered off; the datasheet says the worst-case current
into either of the sense pins is 10uA (which is fine). What it doesn't
say is whether this still holds with the Vcc pin tied to GND - the 5V
rail powering this pin also powers a bunch of other stuff, which will
have a fairly-low resistance path to GND when it's off.

I've been bitten like this before with protection diodes etc which
aren't shown on the equivalent circuit,

It's pretty clear that there are no protection diodes...

"INDEPENDENT SUPPLY AND INPUT COMMONMODE VOLTAGES"

anybody have any experience with
this part? The fall-back plan is to isolate the 138 inputs with a pair
of PMOS's, but it'd be a lot neater if this wasn't necessary.

TIA

R.

About the only way you're going to know is to try it. I would hazard
a guess that the only sneak-path might be the output (when VDD is
removed)... put an analog switch in series with the emitter (output).

...Jim Thompson
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