Precise current sesnse amplifier.

I have current shunt 10mOhm on Low side and current though it for example 0-15A.
And need sense and amplifi it 20x 30x 60x.

My first solution

On first stage One Amplifier from AD8672 as differencial Amplifier with Gain 5x
On second stage second Amplifier from AD8672 as non inverting Amplifier with Gain 4x 6x 12x. Gain switching is realized over small MOSFET transistor and On/Off resistors on GND in feedback.

AD8672 have
Dual-supply operation: ±5 V
Very low noise: 2.8 nV/√Hz, 77 nV p-p
Wide bandwidth: 10 MHz
Offset Voltage 20uV
max gaiun is 12 bandwith is always better than 830kHz

main disadvantage
- high demands on resistances in differential amplifier


Maybe you would recommend other amplifiers or otherwise designed switching amplification?

If the shunt is on the low side, why do you need a differential amp?

Because he wants to measure accurately,
even on a low side can be behind a shunt any pcb, wire, connector and that all have any resistence and therefore, a differential amplifier should also be used on the low side.

Yeah a proper 4 terminal shunt with differential sensing is the best you can do.

What's your actual spec in terms of accuracy and bandwidth? This is DC only? I've designed to quite a similar requirement. Here are some notes:

-OPA388 is one of the higher bandwidth zero drift amplifiers available.
-Other good amplifiers in terms of offset plus bandwidth: OPA8610, OPA140, OPA727, OPA192
-If you don't need bandwidth better choices should be available at <5Mhz
-You'll need matched quad resistor packs to form a precision custom differential amplifier stage. See LT5400 and ACAS
-Forming precision switched gain stages presents another set of challenges and may demand matched resistor dividers (ACAS and various SOT-23 matched packs).
-Alternatively there are a handful of precision PGA IC's such as AD8251 which add a lot of value with their built in opamps, switches and matched resistors.

carpenter said:

Because he wants to measure accurately,
even on a low side can be behind a shunt any pcb, wire, connector and that all have any resistence and therefore, a differential amplifier should also be used on the low side.

Click to expand...

Okay, but you don't necessarily need a dedicated differential amp to do that.
You can use a standard inverting amp configuration with gain, with only a possible small loss in accuracy.
Just connect the resistor input directly to the minus side of the shunt with the other (+) input directly to the plus side of the shunt.
The shunt signal will then be amplified by the amp gain while the voltage drop from any stray resistance will be amplified by only a gain of one.
In most cases the unamplified voltage from the stray resistance will be negligible compared to the amplified voltage from the shunt.

You have to look at it this way.
For example 10mOm shunt and between - of shunt and GND is 20cm Cu wive 2,5mm2
For 20C we have

Rshunt 10,00 mOhm
Rwire 1,53 mOhm

for 40C we have

Rshunt 10,00 mOhm
Rwire 1,65 mOhm

J see total resistance changed by 1.2% of Rshunt only for seltheating of the wire :thinker:

carpenter said:

You have to look at it this way.
For example 10mOm shunt and between - of shunt and GND is 20cm Cu wive 2,5mm2
For 20C we have

Rshunt 10,00 mOhm
Rwire 1,53 mOhm

for 40C we have

Rshunt 10,00 mOhm
Rwire 1,65 mOhm

J see total resistance changed by 1.2% of Rshunt only for seltheating of the wire :thinker:

Click to expand...

Okay.
But we are monitoring only Rshunt, not Rwire.
Any Rwire voltage is only amplified by a gain of 1 for my suggested circuit, not the gain for the shunt voltage.

Hi,

I did a lot high precision, low noise low side current measurement projects.

If I remember right, then I never used an instrumemtation amplifier nor a true differential amplifier.
I often use layout PCB layout techniques to ensure high precision, but use relative low ohmic feedback path to ensure low noise.
A true differential amplifier introduces more noise than a simple inverting or non inverting amplifier.

But it's the decision of the designer to decide the best solution for his application.

Klaus