ADS1294: ADS1294 Schematic

Anonymous

Part Number: ADS1294
Other Parts Discussed in Thread: ADS1191

Hello,

I would like to use the ADS1294 for my measurements in the future. I have designed a schematic for it. (see picture)
Could you take a general look at it and tell me if I can design the circuit like this? I would also like to know the best values for resistors R1-R8 (5kOhm?), capacitors C1-C4 and capacitors C5-C12 (4.7nF?) for very sensitive measurements in the µV range. Also, what values do you recommend for the resistors of the digital interfaces to avoid reflections? (R9-R12)
Are the other values well chosen?
The unused or non-existent channels (5-8) of the ADSS1294 are floating, is that correct?
Is the logic level at 1.8V+0.2 like the ADS1191 if I supply DVDD with 1.8V?

Of course I separate AVSS and DGND in the PCB layout later.

over 5 years ago

0 Alexander Smith over 5 years ago

TI__Mastermind 49041 points

Hi Hendrick,

I would get rid of the differential caps (C1-C4) and just use C5-C12. The size of these components is dependent on input frequency, not input magnitude. Assuming that you're doing ECG measurements with this circuit, 22,1kohm and 4.7pF will be fine. This cutoff frequency created by this filter is 1/2*pi*R*C.

Digital resistors shouldn't be necessary as this is a relatively low speed application.

Unused input channels should be tied to AVDD.

Logic level will be DVDD +/-a few hundred milivolts depending on where you're looking. This can be seen on page 16 of the datasheet.

The schematic is kind of hard to read for the bottom left and upper right. In the future, you may want to spread things out a little more to make is easier to trace the connections. GNDs should always be pointed down.

It looks like AVDD_3 and AVDD_4 are not connected to AVDD.

0 Anonymous over 5 years ago in reply to Alexander Smith

Guru 17045 points

Thank you.

Right, in the beginning it's just a low pass...
That means, if I select for example R=3.4kOhm and C = 1µF, then I would get rid of my 50Hz interference from the supply.

0 Alexander Smith over 5 years ago in reply to Anonymous

TI__Mastermind 49041 points

Hi Hendrik,

Do you mean R=3.4k and C = 1uF for the low pass filter?

That's a very aggressive cutoff frequency. The low pass filter is typically designed to have a cutoff around 10x the frequency of the input signal. If you set the cutoff to be that low, minute mismatches in the R & C components will attenuate the lines differently resulting in a differential offset. This is where the common-mode rejection from the input amplifiers & using the RLD to bias the signal come into play.

0 Anonymous over 4 years ago in reply to Alexander Smith

Guru 17045 points

Hello,

Sorry, I forgot to answer.
I have now opted for a cut-off frequency of about 340 Hz.

My device obviously also works correctly, I can capture data, i.e. noise in general or read data via my signal generator or the ID register etc. correctly.
However, the data is not yet perfect. If I connect my signal generator for a 20Hz and 10mV sine wave directly to the electrodes, what basic settings do you recommend for gain, data rate, lead-off, etc.?
When I analyze the numerical values, I very often have 0 0 0 0 or -1 -1 -1 -1 between two recorded values, do you have any idea why?

Best regards,

Hendrik

0 Alexander Smith over 4 years ago in reply to Anonymous

TI__Mastermind 49041 points

Hi Hendrick,

For gain, you want to take advantage of the full scale range.

This might help: https://e2e.ti.com/support/data-converters/f/73/t/379380?tisearch=e2e-quicksearch&keymatch=ads1298%20analog%20inputs

For data rate, you want to find the trade-off between output sample speed and OSR that you are happy with.

These can both be seen in the datasheet.

For now, keep lead off and such turned off until you get the basic things working.

Most likely, you are outside the common-mode or input range of the device when you get these readings.

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ADS1294: ADS1294 Schematic