AD7731: sinking current

Document created by analog-archivist Employee on Feb 23, 2016
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1 - I cannot see, from the documentation, what is the maximum supply/sinking
current on the digital pins (DIN, DOUT, SCK...) of the circuit. I wish to
interface the AD7731 to th microcontroller board/bus with  optocouplers, but I
do not know if the AD can handle driving their LEDs directly.

2 - For another circuit, an weighing application, I have a bridge that has a
full-scale output of 2 mV. Having tested it, there is too much noise. So, I
need to amplify the signal., and I thought of using an Instrumentation
Amplifier. The AD623 seems a good choice. (Not too much noise, and single
supply).

    But the problem is, the output (reference) of the In-Amp is referred to
ground, and I need to refer it to a higher voltage, because of the limits of
the AD7731.
I suppose I cannot connect it directly and only to the input of the converter,
because it needs a low impedance circuit.

I thought I could connect the reference to the AD780 (2.5 V), and these to the
input, but I do not know if this can be done, or what it means in terms of
accuracy (and safety) to the AD780 or the rest of the circuit.
    I need to extract 18 bit accurate accurate data, so noise is a concern
here.

 

1 - The digital outputs are not tested for their maximum source / sink
capabilities. Digital outputs in general are specified to guarantee particular
voltage high and low levels for a given source / sink current. In the case of
the AD7731 voltage outputs, we guarantee valid logic high levels which the
output is sourcing 200uA (5V supply) or 100uA (3V supply), we guarantee valid
logic low levels when the output is sinking 800uA (5V) or 100uA (3V).

If you require the outputs to sink and source more current than is quoted, the
output levels cannot be guaranteed and the outputs should be buffered. In
general, I would expect that digital outputs of the AD7731 cannot drive the
diodes directly and you should include a buffer.

Also note that the output of a opto-isolator generally has a very slow rise and
fall time and a Schmitt trigger will often be required to make the logic
transitions sharper. Only the SCLK input  on the AD7731 includes a Schmitt
trigger on the input.

2 - The output of the AD623 can be referenced to any voltage within supply
range using the REF pin. It is quite acceptable to connect REF to the AD780
2.5V reference.
However, it will be very difficult / impossible to achieve 18 bit resolution
from a 2mV full-scale signal, using the AD623 to amplify the signal. To achieve
18 bit resolution from a 2mV full-scale, 1LSB is 7.6nV. To achieve 18 bit
resolution from the AD7730 you need to gain up the input to the +/-80mV range
(gain of 160).

The AD623 has an input referred noise of 35nV/rt(Hz), therefore in a 3Hz
bandwidth the 60nV.

The AD620 has significantly lower noise at 9nV/rt(Hz) but it needs dual
supplies. Remember also that the Johnson noise of a 1k resistor is 4nV/rt(Hz)!

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