Electronic – How to calculate the maximum current rating of an ammeter manganin shunt

I am glad you linked directly to the EC4-250-NO Nitric Oxide Electrochemical Sensor datasheet.

That datasheet specified an applied potential of +300 mV and mentions "Operating circuit ... see Electrochemical Toxic Sensor Application Note". I'm assuming that's alluding to the "ev2: Electrochemical Sensors Application Note 2: Design of Electronics for Electrochemical Gas Sensors". (And not alluding to the similar-sounding "e2v: Electrochemical Sensors Application Note 2: Support Electronics for Toxic Electrochemical Sensors" which assumes zero bias voltage, and so won't work with this sensor, since this sensor requires a 300 mV bias voltage). That app note specifically points out (in Table 1) that the EC4-250-NO is designed to have the Vsense electrode +300 mV positive relative to the Vref electrode.

You are trying to build something functionally equivalent to "Figure 4 –Biased Sensor Circuit with Single Power Rail" from that datasheet, right?

The LM285 datasheet guarantees that, if some current Ir in a given range is forced through the LM285, the voltage is a high-precision approximation to 2.5 V. According to page 3 of the the datasheet, that range is Imin == 12 uA <= Ir <= 1 mA for 1 mV precision, up to 20 mA for 20 mV precision.

You need to pick Rdis to set Ir in that given range. So we have

Vcc == 5.0 V ---Rdis--+-- Ir through shunt diode ---GND
                      |
                      +-- 2.5 V reference voltage (set by shunt diode)
                      |
                      +-- 2k2 Ohm + 300R Ohm ------GND

ignoring the insignificant bias leakage in the op amps and insignificant leakage in the capacitors.

So we use Ohm's law to calculate 1 mA through the resistors to GND, plus 12 uA <= Ir <= 1 mA through the shunt resistor, to give the total current going through Rdis

Since we know Rdis has about 2.5 V across it, when the circuit is operating correctly, Then we use Ohm's law to calculate Rdis. I get

1.25 kOhm <= Rdis <= 2.47 kOhm.

p.s.: Clear, easy-to-understand schematics are surprisingly difficult to draw, and yet they make it much easier for people to help you.

In the future, you might consider:

When I use a voltage reference diode on my schematics, I prefer to use a symbol that looks more like Figure 2 in the LM285 2.5V reference diode datasheet rather than a mysterious empty box.

Also, when I break up a schematic across multiple sheets, rather than "2.5 V" on one and "Virtual_GND" on the other, I prefer to give a net the same name on every sheet it is used.

p. p.s.:

Is there some reason you want to design and build from scratch your own circuit, rather than using an off-the-shelf, ready-to-go PCB already designed and built for that specific sensor?

Such as, for example,

• Gas Sensor Evaluation Kit from e2v (and its user guide )
• SensorBox, the EveryAware mobile platform
• CN0234: Single Supply, Micropower Toxic Gas Detector Using an Electrochemical Sensor
• Electrochemical Gas Sensor Transmitters

A single-ended reading from the perspective of your micro is the way to go. Use an accurate analog circuit to compute the difference and quantize that difference with a single ADC channel to get your reading.

I suggest using a smaller shunt (something that will produce 10mV at your maximum load) and a part from the INA210 family - these parts are highly accurate and work in both the high-side and low-side.

If you don't end up with the exact gain you want, you can simply voltage-divide the output of the INA21x and feed that into your ADC input.