Original question:
Hi team,
Below is the schematics I worked with customer. Could you kindly review it? Thanks.
1）AVDD1=AVDD2=+2.5V；AVSS=-2.5V；IOVDD=+3.3V；VREF=+2.048V；
2）analog input range is -1.5---+1.5V；
3）REF is +2.048V from REF70；
4）High speed mode；
5）External clock is 25.6MHz.
Hello Jerry,
Here are my recommendations.
1. Input capacitor C21 is an X7R ceramic. This will work for DC signals, but for any AC signals, the capacitor will increase distortion due to the voltage coefficient of the X7R material. I suggest using NP0/C0G ceramic. You can typically get these in values up to 100nF.
2. You should add a 3ohm resistor between the AVDD1 pin 20 and the external bypass capacitor to improve ADC performance.
3. I suggest adding a 1uF bypass cap directly between AVDD1 and AVSS, and also a 1uF cap between AVDD2 and AVSS. This will help with noise.
4. Connect the thermal pad pin21 to AVSS, not to DGND.
5. For the input amplifier, THP210, I suggest operating from the same +/-2.5V supplies as the ADS127L11. This will prevent the possibility of damage to the ADS127L11 inputs if the THP210 outputs go to the supply rails. Also, the supply resistors for the THP210, R2 and R12, should be reduced to 10ohm or less for good performance.
6. I do not think the configuration that you have used will be stable. For a 2nd order low pass filter, I suggest you use the filter topology in Figure 3-1 of the ADS127L11+THP210 application note:
THP210 Driving the ADS127L11 (ti.com)
7. For the reference, another option is to use the OPA192 and operate from the same +/-2.5V supply as the ADS127L11. If you choose to change amplifiers, the compensation components may need adjusted.
Please let me know if you have additional questions.
Regards,
Keith Nicholas
Precision ADC Applications
