ADSP-SC589_EZ-Board Analog input noise

Also, can you explain why you are connecting a 600ohm resistor between the ADC input pin and GND? 

-Matt

I will try the POST, thanks for the suggestion.

The reason I "shorted" the input with 600 ohms was a part of the troubleshooting process, to eliminate the risk that the noise signal would come from the outside.

If I don't connect anything to the input port it is the same as connecting a source with "infinite" output impedance. In my particular case I get no 3kHz noise, only normal random white noise.

If I connect my normal signal source, I get the 3kHz noise. How can I be sure the signal is not coming from my own signal source?

If I want to connect an absolutely "quiet" signal source, I connect an RCA connector without cable, with 600 Ohms directly connected between signal pin and GND, mimicking a zero signal source with a realistic output impedance. Zero=0 ohms, a real SHORT from input pin to GND has a risk of introducing other problems such as parasitic ground loops, dependent on how the input circuit layout is done. The to select the impedance value here is somewhat arbitrary, 50 Ohms or 600 Ohms are common output impedances of signal sources. But preferably not zero.

Now I have made sure that I have no signal coming from the outside, any signal on the output is generated internally in the analog input stage.

The fact that I get "no noise" with an open input circuit,  and 1mV 3kHz with 600 Ohm connected, indicates that the input noise circuit might look something like this:

Without the input resistor, the input noise voltage sources Vn1 and Vn2 have no effect. With the resistor connected, the input is a closed circuit with Vn1-Vn2 as the input signal, amplified by the input stage so that it appears at the output. Vn1 and Vn2 may be different, correlated or non-correlated.

Does this explain why?

BN

Thanks for the additional information! I will review with some other engineers.

BTW: when you did these measurements, was a JTAG debugger connected to the SC589-EZKIT? Several engineers have told me that having a JTAG debugger connected can cause noise on that board.

-Matt

The noise is present also in a "booted runtime situation" where the EZ board is mounted in a metal enclosure with the cover on, far away form any JTAG debugger, SigmaStudio project download devices or USB cables.

I have measured both while debugging and in the above described configuration, and there is not much difference.

BN 

Oscilloscope screens of 1kHz1mV in, 0mVin, and Disconnected (open input).

Some time have passed now, I wonder if I can expect any answers or suggestions from your team?

/BN

Some time have passed now, I wonder if I can expect any answers or suggestions from your team?

/BN

Did you run all of the Power On Self Tests on this board? And did they all pass?

-Matt

I could not do all tests, since I lack the necessary cables.

Standard loop test passed.

LED test "passed", but.no LED10/11/12 activity.

UART test passed

HADC test passed.

CAN not performed, no cable.

SPDIF coax test passed.

SD card test passed.

INA3221 test failed:

INA3221 Chan 1 single shunt voltage 33760uV not in range

INA3221 Chan 1 continuous shunt voltage 33360uV not in range

INA3221 Chan 1 continuous shunt voltage 33240uV not in range

Si5356A Clk Gen: passed 1  failed 0

SPI EEPROM Test passed

ADC/DAC RED Channels  !!!!!FAILED!!!!!FAILED!!!!!Failed channel: 3

SPI Flash Test passed

Firmware test passed

Firmware version: 1.0
Built on Sep 20 2021, at 19:54:48

ETHERNET_100MBPS no cable
ETHERNET_BRD_TO_BRD_CONT no cable, no 2nd board
ETHERNET_BRD_TO_BRD no cable, no 2nd board

ADC/DAC WHITE Channels  !!!!!FAILED!!!!!FAILED!!!!!Failed channel: 0

Loop flag test passed.

I have to say it is difficult to decode the picture of the analog ADC/DAC test cables. Are really two DAC outputs connected together and then going in to a ADC input? I find this highly unusual. Anyway, I tried to make the analog test cables as I understand the picture, but the ADC/DAC tests all failed.

/BN

Ok, I realise that I will never get any support on this issue. (PCB layout mistake not admitted?)

What I did to solve it was this:

I changed my SigmaStudio and CCES projects to allow two more analog inputs, now four instead of two. I made a diff amp input in the SigmaStudio project using subtract modules, with two analog inputs per channel instead of one single ended. I connected two RCA Phono contacts "shorted" with 620 Ohm resistors to the new analog diff "negative" inputs. I had carefully measured my analog source output impedance to 620 Ohms. After this my analog input channels were silent.

/BN