Yes, that's a good way of doing it, if the effect of the measurement resistors is indeed negligible. That could be a problem with low voltages: you want to have a resistance high enough to measure at least several tens of mV, but if that branch has only 100 mV between nodes then the resistor may affect the circuit's operation.
But often you don't need a shunt resistor, if you already have a resistor in the branch you can measure across that.
Note that for AC measurements the current you measure will always be in phase with the branch's voltage, and that may be quite different from the current's real phase. For instance, if you want to measure the current through a capacitor that will be 90° out of phase with the voltage, but what you measure across the resistor will have 0° phase shift.
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,
Best Answer
Your shunt resistor is way too high, to measure properly, you need a low value resistor and a differential amplifier.
In your test setup, the voltage drop on the shunt resistor might be too high for the component to operate properly.
There are off the shelf products that implement this capability.