ADS124S08: Current Sources

Hi Uwe,

nice to hear from you again. I am glad the ADS124S08 suits your application.

Yes, AINCOM can be used as an IDAC output as you have shown above.
Your implementation should work, but please add RC input filters for filtering and input protection purposes.
Also the inputs which are used as IDAC outputs most likely need some protection in case the ADC can see any overvoltages.

Gruß,

Hi Joachim,
thank you for quick reply.
Yes, we will use RC filters, but I have not drawn them. Here is another question: We already use the ADS1248 with RC filters and the component values from the application report SBAA201 (see 3.2 Noise Performance ...). In this circuit capacitors with 51nF and 5nF are recommended. On the evaluation board of the ADS124S08, the values are 4700pF and 470pF. Since the resistors also change from 6.04kΩ to 4.12kΩ, the filter corner frequency is now about a factor of 15 higher. What is the reason for the different component values ?
For the protection of the IDAC outputs a Schottky Diode like BAS70-7-F on the Eval board is sufficient ?
Best regards,
Uwe

Hi Uwe,

if you have been happy with the RC filter values on your ADS1248 design, you can use the same values with the ADS124S08 design as well.
There is no real right or wrong when it comes to choosing the R and C values.

Following are some of my guidelines for choosing them:

• Select the value of the input resistors such that at the maximum overvoltage that the analog inputs can see, the current into the ADC pins is limited to <10mA.
• Select the differential capacitor to create a corner frequency about 5-10x larger than the data rate.
  It would be nice to get at least >=20dB attenuation at the ADC's internal modulator frequency.
• The common-mode filter capacitors should then be 10-20x smaller than the differential capacitor.

Matching the RC filter corner frequencies on the analog and reference inputs as described in the application note that you referenced is good practice. However, my opinion is, when you are using a low-side reference resistor for RTD measurements, a differential capacitor across the reference resistor is most likely sufficient. You could leave the series resistors and the common-mode capacitors out. Unless you also need to protect the REFP input from overvoltages. In that case a series resistor would be beneficial again.

To be honest, we still have to evaluate ourselves what the best method is to protect the inputs where the IDACs are routed to.
Using a diode and/or again a series resistor are two options. With a series resistor you will need to make sure that the voltage drop across it is not too large so that the IDAC still has enough headroom to operate.

In the end you could use whatever external circuit you used on your ADS1248 design if you have been happy with it.

Gruß,

Hi Uwe,

I wanted to make an additional comment.
When switching between the different RTD measurements, you need to make sure the analog signal settles fast enough. If you see that the settling time is too long for your system, then you should decrease the capacitor values.
You can easily see if you have settling issues by collecting multiple consecutive readings from one RTD. In case the settling time is too long, then the first few readings after you switched to another RTD will be off from your final value.

Gruß,

Hi Joachim,
thank you for your advice. That helped me very much.
Regards,
Uwe