Using RCV420 for other current ranges

Hi David,

While we could likely adjust the RCV420 to accept slightly different input current spans than 0-20mA, the range you've mentioned is significantly different and we should probably look at other solutions.  

Have you considered using a transimpedance amplifier with a negative offset? See below.  The circuit may not work depending on how your sensor needs to be connected, but provide a little more information and we'll work to figure something out.

Thanks Colin for the clarification. That was my initial thought just by my initial calculations and looking more at the adjustment features of the device, there was not too much documentation on what the capabilities was so for it. The temp probe that we are using is the analog devices AC2626. It is a 2-wire device, 1 for 24V PWR and 1 for SIGNAL.

I have not looked at a transimpedance amplifier. Current, it is implemented as shunt resistor of 10k to get a voltage range between 2.982V to 4.322V and then isolating the signal using an ISO122. Then goes into a 2nd order LP filter, then to a DAQ system. I assume the DAQ is reading the direct range above.

Is there an advantage between just a shunt resistor and then using the transimpedance amplifier?

Thanks

Hi David,

Either circuit will work fine for the current measurement, it probably depends more on how the sensor needs to be biased for proper operation.

Hi Collin:

The temp sensor gets 24V on it's supply and spits back a range of 218.uA to 423.2uA (-55C to +150C). No special biasing is needed for operation.

Instead of 0-5V, we have decided for a 0-10V output if possible.

Thanks
-David

Hi Colin:

Thanks this looks fairly good. I took a look at it in TINA-TI. I tried modifying so that instead of a 1k shunt resistor, I was seeing what would happen with a 10k. With that, I get a range from 2.182 to 4.232V. I would assume then the offset needs to be ~2.182 voltage and the gain would be at 4.87 (0-10V scale / 2.05V scale). But it looks like there is some issues with it, in the sense that my output is from 2.18-12.17V. RF = 414.09k, and RG = 107k

As I adjust the offset (just used a source rather than resistor divider to test it), I need to be around 2.74.

Does that sound right? It seems like it is very sensitive in what the offset it, and maybe the parameters of the OP AMP are affecting it?

Any explanation of this would be helpful!

Thanks

Hi David,

The circuit I proposed provides a positive gain and negative offset to the sensor output to convert the ~218.2uA - 423.2uA to a 0-5V output.  Terminating the sensor into a 10k load results in a much bigger offset (218.2mV vs. 2.182V) which makes the circuit component sizing a little easier. 

If you want to change the input signal to the design then you'll need to change all of the components, (R2, R3, RG, RF), not just RG and RF.  I also don't recommend using a  non-standard reference voltage (i.e. 2.74V) because it will be hard to generate accurately.  It's better to start with a standard reference voltage (2.5, 4.096, 5, 10, etc.) and adjust the R2, R3, RG and RF component values to achieve the desired output range.