Current Shunt Amplifier Problem (MAX49921)

[greg.ives]

Hi All,

I'm currently trying out the evaluation kit for the MAX49921 from Maxim, and I'm having a few problems with it. First thing to note is that my board appears to be an issue a board, but all the documentation on the Maxim website is for the issue b variant.

So, the problem I'm having with it, is it seems to be unstable/unreliable when drawing small currents through the shunt resistor. The shunt resistor I'm using is a Murata 3020-01096-0 (1mohm), and I'm using a TTI bench supply to supply power (28V), and a Keysight n6700c dc load mainframe with a 100W module.

As delivered, the device didn't produce an output until 900mA was being drawn through the shunt. Interestingly though, once it was outputting, the current could be stepped down and the device would continue to work down to 10mA.

After some experimentation, I've found adding filtering to the input and output seems to help. The best results I've achieved so far have been with the following values;

R3, R4: 1k6
R7: 200R
C3, C4, C9: 10nF

However, even with filtering, the behaviour starting from a low current still isn't right. The device still won't respond to 10mA, and at 20mA, the device does work however it falls over if the current is increased without turning off the DC load in between. At higher currents, 100mA+, the device continues to work when the current is stepped up.

A couple of other things to note:

The amplifier is being powered at 5V from the second output of the bench supply, and the 0V of both outputs are linked.
When adding filter components, I've cut tracks as necessary to make sure components aren't shorted out.
I'm using screened twisted pair wire (KU05) to connect the board to the shunt resistor.

A link to the datasheet..
https://datasheets.maximintegrated.com/en/ds/MAX49921EVKIT.pdf

Any help/ideas would be greatly appreciated, I'm all out of ideas! 

Cheers,
Greg

[StillTrying]

How are you measuring the small output voltage, 10mA x 1mR x 50 is only 0.5mV output, and the Output Voltage Swing Low doesn't guarantee anything useful below sround 4mV.
You might have to supply a small constant current offset to keep the output a few mV above 0V and the opamp working.

https://datasheets.maximintegrated.com/en/ds/MAX49921.pdf

[greg.ives]

I've been using an Agilent 34410A DMM to measure the output, and another to check the current being drawn through the shunt resistor. Even with no load, it does tend to measure around 0.5mV - I suspected the input offset voltage might be responsible for this. If you test in a particular sequence, the output rises to 0.8mV when 10mA is drawn through the shunt and returns to 0.5mV when you turn off the dc load, although it's beginning to sound like this might be more by luck than design.

Where are you thinking of applying the offset current, through the shunt?

Choosing a bigger shunt isn't out of the question, it just has to be able to withstand cases where we might be drawing 20A.

[StillTrying]

I still think you're expecting too much of the opamp with it's output down at 0.5mV. the data sheet doesn't show much down there.

"Choosing a bigger shunt isn't out of the question, it just has to be able to withstand cases where we might be drawing 20A."

Decide on the highest current you want to measure, you can measure a constant 20A ATM because it's only 1V on the output, and 20mV 0.4W in the shunt.

IF the 28V is fixed, something like that might work, but I think if you're wanting 0mA to 20mA resolution you'll end up with a larger shunt resistance.

[ConKbot]

Got any negative voltage rails? A pull down resistor on the output to just about any negative voltage will help extend your negative swing closer to ground. Limit the current so it's not introducing a significant offset on the output resistance, of the amp.

[Doctorandus_P]

The simplest solution is to not use MAXIM parts.
Then you can never have a problem with them either.