ADS1118: Temperature monitoring using ADS1118

Purvi,


Is this a question more about the INA826 or about the ADS1118? If it's more about the INA826, then it might be worthwhile posting it again in the Precision Amplifiers Forum also. Regardless, I'll give you a few thoughts.

First, I highly recommend looking at the ADS1118EVM User's Guide. The evaluation module is designed to run an application measuring a thermocouple. Depending on the type of thermocouple you decide to use in the end, it may have the exact circuit you want. If you can, it might be good to get one of these modules and take data with it. Note that the module does not use any gain stage and the thermocouple connects to the ADC with some RC filtering. There is gain in the ADC listed as changes in the full scale range. The largest gain is a full scale range of ±0.256mV, which might be smaller than what you desire.

Looking at the INA826, the offset is 150uV max and a gain error of 0.15% max. Note that the gain error of this stage is also determined by the RG resistance. If you use a 1% resistor, you'll probably have about a 1% gain error. Note that if you're looking to do something with ±0.1°C error, you are probably already at an error larger than that and you'll need to do large amount of calibration.

Compare that to the ADS1118, where the offset error is typically 10uV or so with a gain error is 0.15% max. If you use only the ADS1118, you might lose resolution because you're not utilizing the full input range, but you don't add on any additional errors for offset and gain error. It will have less error and you would only need to calibrate the errors of only one device.

I would also note that the line noise may not always be a common-mode signal. Long thermocouple wires may act as an antenna and pick up line noise that would be seen as differential.

What type of thermocouple do you intend on using? What temperature range are you expecting? Those two questions will determine what resolution you will have if you use the ADS1118 without the INA826.

How do you intend to connect the ADS1118 to four inputs? Are you planning on referencing four INA826 to ground and make four single ended measurements with the ADS1118? How do you want to do a cold-junction measurement?

If you really want to do a precision temperature measurement, I think that you'll need to use something like the ADS1220. It is similar in function, but has a 24-bit output. You could conceivably reference all thermocouples to ground and make four measurements, but you would need to be very careful in layout, making sure you have a clean ground from which the thermocouples are referenced. Personally, I would use two ADS1220 to measure four thermocouples.

Additionally, the ADS1248 and the new ADS124S08 are good candidates for this project as well. If you have any questions about those devices, please post back.


Joseph Wu

Thanks Joseph,

The type of the thermocouple would be either J type or K type.The temperature range expected is from 0 degree to max 1300 degrees.

I refered to no of Instrumentation amplifier and it seems that most of them has offset voltage of few micro volts.So if I am planning to use he instrumentation amplifier between thermocouple and ADS1118 the desired accuracy won't be achievable.

But in case to provide good rejection of 50Hz and 60Hz noise,I think instrumentation amplifier would help.

If it is possible to reject 50Hz and 60Hz noise using ADS1118,then circuit implemented in ADS1118EVM can be used.

But still the question is ADS1118 enough capable of rejecting 50Hz and 60Hz noise?

Regards,
Purvi

In order to use single piece of ADS1118, if I am using TS3A5017DR multiplxer in between thermocouple and ADS1118,then would it create a problem in terms of accuracy or noise rejection?

Purvi,

The ADS1118 has some minor amount of 50/60Hz rejection. However, like other similar ADCs, this rejection depends on the data rate. First, let's look at the ADS1118 datasheet at page 14. Figure 34 shows the frequency response of the digital filter for the device running at 8SPS.

This will have the best 50/60Hz rejection of any data rate for this device. It's hard to see, but the rejection will likely be about 25-30dB. This device wasn't designed to have a good notch at 50/60Hz to reject that noise.

As an alternate example, let's look at the ADS1220 at 20SPS, with a simultaneous 50/60Hz frequency rejection. A frequency response of the digital filter is shown on page 25 in figures 46 and 47. With this setting with the filter and data rate, the 50/60Hz rejection is greater than 80dB. This is a large improvement over the ADS1118.

I'm still not sure how you think the INA will help you. It may have a good common-mode rejection, but most ADCs already have a good common-mode rejection. What you want is a normal-mode rejection of signals at 50 and 60Hz.

As for your comment about using a mux, I generally don't see a problem using a mux for measuring a thermocouple. The mux series resistance isn't a problem because there's no excitation current necessary. On-resistances and parasitic leakages.

Joseph Wu

Thanks Joseph,

In order to measure 4 thermocouple outputs,if i am using 1 piece of TS3A5017DR multiplexer and 1 piece of ADS1118 or ADS1220 and the flow of circuit is as follows:

thermocouple -->Mux-->ADS1118 or ADS1220

Let's say if 50Hz noise arrives at the input,so at the very first stage i.e MUX, won't it get saturate?

ADC will surely attenuate the noise,but as MUX gets saturate it would provide incorrect input to ADC and ultimately incorrect temperature data would be read.

I am not sure whether MUX will create a problem or not in terms of 50Hz noise rejection.

Regards,

Purvi