This is the circuit I posted six or seven years ago for high side
current measurement. There is nothing novel or exciting about it:
        0.1R
24in-*--\/\/\/\--*---24out
     |           |
     \           \
     / 1M    1M  /
     \           \
     |           |     10M
     |           *----/\/\/\/----|
     |           |    |\         |
     |           *----|-\        |
     |           |    |  >-------*----  5v=3D5A
     *-------------*--|+/
     |           | |  |/
     \           \ \
     / 100K  100K/ / 10M
     \           \ \
     |           | |
  ---*-----------*-*-- ground

The main thing to remember in this and any other instrumentation
circuit, NO CARBON RESISTORS! Carbon resistors are crap. If you use 1%
metal film resistors that you've hand selected, the circuit is pretty
accurate. If you are mass-producing it, use 0.1% resistors so you don't
have to hand-select them. The shunt resistor (the low ohm one at the
top) doesn't need to be as fantastic as the others because its error is
not amplified disproportionately. The opamp needs to have output that is
valid to ground, and even so, if you are running single-supply, the
output can never actually be zero because of the 10M feedback resistor
working against the output impedance of the opamp at 0 volts. It amounts
to fewer than 5 millivolts on a crappy LM324 type opamp. Though I think
when I posted the circuit I was leaning toward an LMC6484 or something
like that.

Cheerful regards,

Bob


On Mon, 30 May 2011 17:08 +1000, "cdb"  wrote:
> Bob Blick has a circuit on an SX forum, I happened across it the other
> day.
>=20
> 7 resistors and one op-amp (rail to rail) high side sensing.
>=20
> V+in -    sense_R ---------- V+out
>=20
> R_div_1a           R_div_2a   =20
> R_div_1b           R_div_2b
>=20
> 0v-------------------------------
>=20
> Op-amp V+ connects to R_div_1 centre tap
> Op-amp V- connects to R_div_2 centre tap=20
>=20
> Op-amp feedback resistor from op_v_out to op_V-
> Resistor from op_V+ to 0v
>=20
> Nominal values:
> R1a,R2a 1M
> R1b,R2b 100K (input voltage dived by 11)
> Feedback R and R to 0v  10M
> Sense resistor 5v/current to be sensed.
>=20
> I notice that TI and Linear use either FET's or BJT's in the feedback
> loop,=20
> byt they have converted from V to I and back to V.

--=20
http://www.fastmail.fm - Send your email first class

--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.