Handling the output of an AD8495

Question

I am building a circuit with two AD8495 thermocouple amplifiers. I chose them because they are simple to get and quite cheap - there's basically no other reason. Now I need to create an interface between the amplifiers and a microcontroller (AVR). The AVR's ADC has 10 bits resolution, and I'd like to use as much of that as I can.

The temperature I am expecting at the thermocouples is up to 300°C. As the AD8495 gives 5mV/°C, the maximum output should be somewhere around 1.4V. I could supply a suitable reference voltage to the ADC to resolve that range, but when the AD8495 has no thermocouple connected (or something else goes wrong), the signal will rise to 5V (supply voltage), which would be too much for the ADC with a low reference voltage.

I see two basic options:

I can clamp the signal to the reference voltage (or some "compatible" voltage using a resistor and a Z diode:

This is simple and cheap, but I think I might mess up the whole measurement with this.

I can also use an OpAmp to amplify the AD8495's output to the full 5V range:

Connected with these options are two equivalent problems:

• I have never build a clamping circuit with a Z diode,
• I have never used an OpAmp before. I picked the TS912 because it's a rail-to-rail, single supply type (and I can easily get it)

While I know how the ideal devices are supposed to work, I also know that the real devices will mess with my measurement a lot.

I would like to achive +/- 5°C accuracy at 280°C. I have not yet done calculations on that, but given the ADC, reference, and AD8495 accuracies, this might be a bit too optimistic. I am aware of that.

Link to datasheet download page

---

Edit 1: The AVR has an internal 2.56V reference which I could use. Unfortunately, it is not very precise (2.4 ... 2.8 V) and I'm not sure how much current it can sink or source. So even if the ADC channels have internal clamping diodes (which would have to clamp to Aref), I need some extra protection or conditioning. Using an external reference will not really solve that problem either.

Edit 2: This is my solution, more or less:

The two AD8495 outputs are connected to an ADC (MCP3426), which has an internal 2.048V reference. That is much more accurate than the AVR's internal reference and tolerates the AD8495's 5V output when no thermocouple is connected. I've added screw terminals for the thermocouples and a 0.1" header. All pin holes are on a 0.1" grid. Layout and routing was quite easy:

There are two short SCL and SDA traces on the bottom side, everything else is ground. I've made the thermocouple traces (left) a bit wider for lower thermal resistance.

Answer 1

An accuracy of +/- 5°C accuracy at 280°C amounts to about 1.8% error. This results in an effective resolution of 6 bits (assuming the full measurement range would end at about 280°C). 10 bits of resolution would result in about +/- 0.28°C accuracy, and 8 bits in about +/- 1°C. So you don't need to worry here (Even when you are not using the full range of the ADC input).

The easiest solution for your overflow problem could be to use Avcc as reference voltage (but then it should be noise-free, precise and stable enough). This reduces your resolution (by half when compared to the internal reference, because it doubles the measurement range), but you have plenty of room there (you use about one fourth of the ADCs input range then, so you get 8 bits of effective resolution over your temperature range).

If you want to improve resolution, use a 3.3V low-noise regulator to create both Vref for the AVR, and Vcc for the AD8459 (it can run with this voltage). That way you can be sure the voltage from the thermocouple amplifier never exceeds the reference voltage.

But you also can use a zener diode to clamp the voltage of the amplifier. When looking at e.g. the ATMega16 data sheet (you did not specify which AVR you use) it has an input resistance am 100MOhm, and states that an input impedance of less than 100kOhm is suggested. So the clamping will not have any effect as long as R1 in the schematic above is small enough. And 10kOhm would be perfectly OK - the amplifier then needs to source 0.5mA.

Using an external ADC is another solution. If you can afford the board space and the additional components, it seems even like the best solution. Look for an ADC with a reference of 2V which also can stand inputs up to its Vcc, then you are fine.

I personally would go with the 3.3V LDO solution. You might need a stable and noise-free reference voltage anyway, so why not using it to solve other problems as well?