increased A/D noise when isolated

Hi Joseph,

To be able to identify which device is responsible for the issue, could you please elaborate more on the issue?

1. Where do you see the noise? At ADC inputs? Only at the ADC output? Or in the PCB after you read through USB?
2. Can you please share any waveform of good and bad signal?
3. What kind of signal that you are feed in to ADC?

Please do share as much information as possible so that the issue can be rootcaused quickly, thanks.

Regards,
Koteshwar Rao

Hi Rao,

thanks for the response.

1. Noise is present bot the ADC input and read-out USB data.

2. Unfortunately I can send waveforms on next week (currently the circuit is not here where i am..).

3. As i mentioned, the MSP has a (test) signal output (TA2) as well (generated by TIMER_A - used as NCO), but similarly noisy if input simply tied to AGND (isolated from USB's DGND)

I would have a question about performance of capacitve isolation:

Whether it offers same effectiveness from aspect of common mode noise as inductive or Photo isolation? Because referring to the digital signal transitions (high frequency content!) the isolating on-chip oxide capacitor (~1pF) exhihibits a limited impedance only (not infinite as an inductive/opto can).

Regards,

Joseph

Hi Joseph,

Thanks for your inputs.

1. If noise is observed at ADC input and at the read-out USB data then it doesn't look to be isolator issue.
   1. If you can share the isolator part number, I can recommend you equivalent or better TI part number that you can also test and check.
2. Sure, you can share the waveform when you get them next week.
3. Understood.

For TI isolators, the isolation capacitors are deep inside the device with many other circuitry preceding them to inputs. The isolation capacitor existence in the device is transparent to external signal inputs. The isolation caps primarily determine the high voltage performance of an isolator along with a few other HV EMC tests. In your application, it doesn't look like you have any high voltage transient to affect isolation barrier.

Since this issue doesn't look to be an isolator issue, I will let the MSP430 team to respond to you. Thanks.

Regards,
Koteshwar Rao

Hi Rao,

You may have right:

the problem is should not caused by the isolator (since no HV signal between the two grounds):  When I touch with a piece of wire the Power transformer secunder winding or one of SPI (especially SCK) signals the noise dramatically increase. First I tried to complement the SN6501 circuitry with RC snubbers (200pF+2KOhm) but there was no effect (or only neglible).

Note, the pre-amp has high impedance (n*100KOhm) signal paths altough solid ground plane used. It seems it is an onboard EMI issue. Unfortunately, ferrite beams start to be effective above 10..30 MHz (my SPI is run on 2MHz and SN6501 similar range).

Waveforms: GND not touched:

GND touched:

Kind rergards,

Joseph

Hi Joseph,

Is it possible to share your PCB design? You can either take screen shot of layers in your CAD tool or the output gerbers (or pdf version of gerbers) that are generated by your CAD tool.

Hi Dennis,

thank your for your reply, I included a layout: IC9 at the upper lefet corner is a frequency divider HC4518 as test signal output. Note,

recently instead of IC9 (I removed it) I use an SN74AHC1G04 drived from TA2 out of MSP430 not the ALCK out. IC8 is a dual OPA (RC4580) acts as an active attenuator+level shifter, IC6 is an analog 8-1 MUX (for attenuation setting).

Regads,

Joseph

Hi Joseph,

When you took the two scope captures, where did you have the ground lead of the probe connected and how long is this ground wire on the probe?

Next, in your first picture showing the noise on the square wave, if you re-program the MSP to stay in LPM3 (no CPU or clocks running) then probe this same point do you see noise and if so does touching the ground plane make a difference?