ADS1147 4 channel diffrential inpuy

Manyank,


First, I generally wouldn't use any inductors in the input path (or even in the power) feeding the ADC. In general, the digital sampling of the ADC will cause some spiking of the currents (in this case the input) which will lead to spiking of the voltage. For your tests, remove the inductors and replace them with shorts.

For debugging purposes, I would also remove the input protection diodes on the front end. These devices can also have leakage currents that cause error.

If you've measured all of the AIN channels, and they measure correctly. I would measure the reference as well. Remember that the ADC is a comparison between the input and the reference value. If there is a change in the measurement, then the source of that can either be the input or the reference. I might also take an oscilloscope measurement of different channels and inputs. Since there are different timings for the ADG1414 and for the ADC measurement, you should makes sure that these are aligned the way you think.

If you are seeing that it takes more time for the AIN6-AIN7 measurement to settle, how long does it take? What is the approximate RC time constant? Are you switching this channel differently than any other channel? If there is a time constant, it might be worth taking off the capacitors on that particular channel.

Incidentally, I would make the differential capacitance to be larger than the common-mode capacitance by an order of magnitude. As an example with the AIN6 and AIN7 channel, use 1uF for C32 and 0.1uF for C31 and C33. This isn't likely your problem, but it's good practice to make sure that the differential noise attenuates before any common mode noise.



Joseph Wu

Thanks for the detailed write-up.

On Saturday, we started debugging by the process of elimination. Inductors were removed and shorted. Filter Caps were also removed, because they were holding up charge when the signal was removed.

Reference voltage measured 2.048V

ADG1414 is only switched once after power-up. So i would think settling time will not be much of an issue and also we noticed the reading error on board where we did not populate ADG1414.

It finally resolved when we removed input protection diodes (SMAJ10CA - 10uA reverse leakage current). Now we are testing the signal accuracy over complete range of 0-20mA.

But, removing input protection diodes makes circuit prone to field failures. Can you suggest any alternate approach for input protection? or may be any other protection diode part number?

Once again many thanks for your time, really appreciate it!

Manyank,

I'm glad you were able to figure out your main problem. It was pointed out to me that there is another issue in your schematic. If you look at the input to AIN6-AIN7, you are measuring a resistor that is connected to your ground. The ADS1248 has a common-mode input range that does not go all the way to ground. If you look at the specification table, the analog inputs are valid when the common mode input is:

As a result, in a PGA gain of 1, the input must be above 0.1V above the ground. You should be able to achieve this by adding an extra resistor below the last resistor, just to raise the stack of resistors that 0.1V. If you want to check it, you can make some measurements with just that channel, recording both the input and reference voltages, and compare them with expected measurements. I think you'll find that there is a small error for just that channel because it violates the input common mode range.

For input protection, what are you trying to achieve with the input protection? Are you trying to meet a certain level of ESD protection, or any particular standard?

Joseph Wu

Manyank,


The series resistance my offer some minor protection, but at 50 Ohms, that might not be high enough to do much good. I've never tried this, but the TPD6E05U06 might work for ESD protection. These devices are used for ESD and surge protection. They have low leakage (<10nA) and low I/O capacitance (~0.5pF), which would likely be fine for your application.

Again, I haven't tried these devices, so you would have to put them in yourself and test them to see how they perform.

I also want to make sure that you do add in an extra resistance to raise the common mode of the last channel AIN6-AIN7. Without this you'll definitely see error in that measurement.


Joseph Wu