measurementoperational-amplifierscalingvoltage measurement
I'm seeking a solution how to do a two point scaling in electronics. I have tried an Op-amp, but it did not work very well. So I wonder if you have some ideas how to do two point scaling from e.g 0.6 V – 3.2 V to 0.0 V – 3.3 V?
When I apply 4 mA, I what the output to be 0.0 V or very close to 0.0 V.
When I apply 20 mA, I want the output to be 3.3 V or very close to 3.3 V.
The Zener below is a 3.6 V Zener.
Is that possible?
You need to respect the amplifier input common mode range (and output range). If your amplifier (I'll assume it's an AD8221 inamp) had large power supply voltages your bridge configuration would be fine. You're putting about 0.7mA through the RTD element, which is reasonable for a Pt100. Your full-scale span will be 100mV, so you need sufficient gain in the inamp to give you full scale span for the ADC. If it's 2.5V, then G = 250.
You might want to use a bit lower than 100 ohms if you want to cover 0 degrees C for sure.
The problem is that the valid range of the AD8221 looks like this with +/-5V supplies:
There may be other instrumentation amp types that can work acceptably with a single 5V supply (you would probably have to decrease the 100R resistor a bit to offset the minimum output above zero). Also note that it can't get to 5V output.
Another option is to create a -5V supply using an ICL7660 or similar part.
Yes, as you suspect the CA3130EZ is not unity gain stable without compensation capacitor(s). On page 3 of the datasheet states that a capacitor (called \$C_c\$) of at least 47pF be placed from pin 1 to pin 8 of the device. \$C_c\$ provides Miller compensation, and reduces bandwidth of the part. Circuits shown with unity gain also typically have a 2kOhm || 0.1uF feedback network, and a 25pF load. See the Bode plot of open loop response on page 14 for details of effects of various values of \$C_c\$ and \$C_L\$.
Best Answer
You're only losing 20% of the range and you have the advantage of a "live zero" which can be used to detect a break in the 4 - 20 mA loop. This is the standard solution taken by industrial PLCs and many of those do that on a 0 - 10 V input by addition of a 250 Ω shunt resistor to give only 5 V at 20 mA. Usually its simplicity wins out over the loss of resolution.
A further advantage in the industrial sensor applications is that sensor faults can be indicated by sending a 3 mA signal, for example.
From the comments:
That's for you to decide. You're using a 12-bit / 4096 step ADC. That's a step resolution of 0.02% of full-scale per step. Losing 20% gives you 0.03% of full-scale per step. You're going to have difficulty getting better than 0.1% shunt resistors - see Vishay for example and noise will introduce further problems. Introducing an op-amp to do the offset properly only makes it more difficult.
Figure 1. It appears that the top and bottom of the STM32 ADC are accessible.
The other option is to offset the ADC VREF- to 20% of VREF+. See page 68 of the datasheet.
Added by OP: