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ADS131M08: REFIN pin maximum load current

Bela Istvanfi
Prodigy 30 points
Part Number: ADS131M08
Other Parts Discussed in Thread: ADS131M06

Dear Colleagues,

this is not a real question, just my experience.

Due to providing the proper common mode voltages to the inputs, I need the half of the reference voltage.

I've thought I can use the REFIN pin as reference output, divide it to half by a resistive divider and then buffer it by an opamp (MCP6V81T) for all other purposes.

I was wrong. This pin cannot tolerate any load. By the way, this is not mentioned in the datasheet, or I just cannot find it.

At the begin, I loaded it with a 2 * 24k resistive divider, and this load was so high ( 50 uA ), that the reference voltage has fallen down to about 50%, 0,6V.

OK, I changed the resistors to 2 * 1MOhm. Now, the reference is 1.1596V, while when completely unloaded (well, 10MOhm by the meter) it is 1.1831V.

This chip is still "virgin", has never seen even a clock signal, nor config data. Power supply is OK, 3.3V each, analog and digital too.

It is hard to imagine, how such a weak reference is able to supply the internal circuitry.

I think, in the next version of the circuit I will use an external reference.

  • +1
    TI__Mastermind 18900 points

    Hi Bela,

    Bela Istvanfi said:
    I've thought I can use the REFIN pin as reference output

    This is a good point. It could be more clear, like a specification saying REFIN output load current = ~50uA (or better 0A typical) in recommended operating conditions section.

    The rest of the context you don't know is that ADS131M0x is a family of devices. The AD131M04 and below don't have a REFIN pin, where ADS131M06 and above do. Given we started with the lower channels first, of course the same reference generation circuit wouldn't be used to for external loads because it was impossible to tap into the reference voltage directly. Again, we don't expect the customer to know this but it should help explain why the device the way it is. 

    Here's a couple of other times this question was asked if you want some more information on the topic:

    So yes, the device designed so that no load current should be taken from the device, but in reality, it can safely do a couple of microamps until it starts to drop out.

    Bela Istvanfi said:
    Due to providing the proper common mode voltages to the inputs, I need the half of the reference voltage.

    I do think its the best practice, but I find you can still get very good performance dividing down AVDD with the appropriate resistor divider ratio and appropriate buffer or filter. The way I see it, dividing the common mode from the reference voltage prevents temperature drift error from one source moving away from another, and some common mode error as the common mode will change. I haven't done the in depth error calculation after calibration but something tells me you'll end up with <10s of ppm error difference between the system where the reference can source the common mode. And as a result, I doubt it will be close to your dominant error source.

    Also, I'd like to point out the recommended operating conditions of the input. As you can see, the inputs can swing below GND. Assuming we're still within the FSR, the ADC will be able to sense those voltages. So, it is common to keep the negative input to GND for something like voltage sensing inputs. 

     

    Best,

    -Cole