Differential amplifier with common voltage far above the rail?

Question

I have a thermocouple with one end connected to +24V. I want to be able to read the voltage (-5mV - +20mV) on it using a 3.3V ADC. The most reasonable solution I see is lowering the voltages using resistor dividers, than two voltage followers and a differential amplifier (like here: https://electronics.stackexchange.com/a/93467/61946).
I could also power the op-amp from a higher voltage rail, though that would mean constraining the output voltage in some other way.
Is there a simpler solution I am missing?

Answer 1

This is a suggestion only. Do a precision voltage-to-current conversion using 25V as your "ground", then do the rest of the work ground referenced. None of the component values I show should be trusted. R1, R2, R4, and R5 set the gain. You need to choose a part for OP1 that works decently with it's common mode voltage at or close to the positive rail -- even with rail-rail amps this is not a hugely common thing.

A bit of theory: M1 doesn't flow current in the gate, so the voltage that you maintain across R4 sets the current. R5 "catches" this current and turns it into a voltage (you want to buffer it immediately with a voltage follower, BTW). R3, C1, and R1 & R2 in parallel compensate the circuit for the FET's gate capacitance (use a wimpy fet with low gate capacitance). The 79L05 provides a regulated rail-5V supply for the high-side circuit.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

EDIT:

The solutions I gave previously do not compensate the cold junction temperature, which is a bit annoying. Compensating for offset is also going to be a chore.

So, in the end I would use something like a MCP9600 which is an I2C thermocouple amplifier. According to its datasheet, the thermocouple inputs common mode range includes GND to VCC, so it is possible to "hang" the chip from +24V as suggested by Tim Wescott, by using a negative voltage regulator having its GND pin tied to 24V and IN pin tied to GND.

Now you have to voltage shift the I2C output of the MCP9600 back to ground, which is annoying... or requires a USB isolator chip. It would be better to use a SPI interface chip, which does not have bidirectional pins, and is therefore a lot more practical to level shift, but I couldn't find one which supports an input common mode range including VCC (MAX31855 will not work).

So it looks like I'd go with MCP9600 + USB isolator.

Previous answer:

If you want a canned, one-chip solution you can use a Current-sense amplifier like AD8210, MAX4376, lots of offerings from most chip vendors.

AD8210 requires 5V supply, but MAX4376 works on 3V. These both support the 24V input common mode you have. Make sure you check the offset and mind your error budget. Check the "Current sense amp" category in mouser or digikey for lots of different models.

As far as "roll your own" goes, I'd do something like this:

^{simulate this circuit – Schematic created using CircuitLab}

This is similar to analogsystemsrf's solution, with a dual matched PNP like MMBT3906 which is very cheap and will avoid temperature mismatch between transistors. Adjust value of R and current to keep the transistor linear at the thermocouple voltage you have. The thermocouple is "V1" here. Adjust R3 for gain. You can use any fancy current mirror you want, Wilson, base current compensation, but the basic idea remains the same: add an offset to the emitter resistors of a current mirror.

Q3 is a current source which injects a current into the mirror. Adjust voltage "REF" and both bottom resistors to a suitable value. I'd use 0.5-1mA bias current, so 1-3k instead of 100R as dwrawn.

Measuring the voltage differentially between the two OUT and OUTN points allows to get rid of the offset due to the bias current. Transistor base currents will still add some offset though.

Note: The resistance of all the wires to the thermocouple is in series with R1.

All solutions which cause current to flow through the thermocouple, like the one above, will create an offset due to voltage drop: thermocouple wire resistance x current. Please make sure this offset is acceptable. Otherwise you will need to use an opamp sitting on the +24V rail as per Tim Wescott.

Answer 3

Thermistors aka temperature sensitive resistors, also used in VCO variants.

They would try to improve the "correspondence" with matched pairs & the thermistor is physically-temperature coupled to the matched pair. (temco is A brand)

Answer 4

How about this

^{simulate this circuit – Schematic created using CircuitLab}

Answer 5

Here is another suggestion:

^{simulate this circuit – Schematic created using CircuitLab}