AD converter. How to test?

[9 volts]

Hi there, is there a way to test functioning on a AD converter.  I am looking at a ADC108s022. Five potentiometers are inputting a voltage to it, but altering the pots (altering the voltage) doesn't seem to affect the unit. All controls are suck on full (10). Thanks

[9 volts]

Well no luck. I replaced the ad converter, but didn’t solve the problem. All potentiometers are still unadjustable  and stuck on 10. It’s a digital rotary effect pedal so still works but can’t adjust balance etc. not sure what else to try.

[Digital Larry]

Please tell us how it is you are clocking/measuring.

From the spec sheet,

https://www.ti.com/lit/ds/symlink/adc108s022.pdf


15 DOUT Digital data output. The output samples are clocked out of this pin on the falling edges of the SCLK pin

If this is part of an existing circult which isn't working, it's going to be quite tough to diagnose without a scope.  Have a scope?

[Rob Strand]

Larry is saying you are reading out 1's (or FF's) because your logic signal levels or timing for reading are wrong.  Clock edges, enable lines, many possibilities.  1's come from reading floating or lines with weak pull-ups.   The cpu/micro will read any junk that's on the pin it doesn't care what the A/D expects.

[ElectricDruid]

When you say "stuck on 10" do you mean that if you test the voltage at the ADC input, you always get a high voltage regardless of pot position?

Or are you talking about the digital output from the ADC?

Have you got a schematic for the circuit you're talking about?

[anotherjim]

I had a glance at the datasheet. It reads one default input unless you write to the register. CS has to go low or its in standby.
I did notice it can be damaged if the digital supply ever goes 300mV higher than the analog.

[9 volts]

Hi there, firstly thanks for the feedback. I'll try to answer most of the questions.
The unit is called a Lester k from electro harmonix. Unfortunately they don't give out schematics.
'Please tell us how it is you are clocking/measuring.' I'm using a very basic oscilloscope (small kit thing and a voltage meter).
As mentioned I replaced the AD converter with a new one, this had no effect on the problem.
Footswitches work (off/on, fast/slow rotary). All potentiometers behave they are flat out (eg 10) So volume is flat out, drive is flat out. When I measure the voltage on the potentiometers they are all working. That is, they adjust a voltage from 0 to 3.3volts. All pots go to the AD converter.

SCLK pin
I could see a pulse looking wave on this pin.
15 DOUT Digital data output
Looked like  3.3v on this pin
DOUT heads to spansion IC (memory)

"When you say "stuck on 10" do you mean that if you test the voltage at the ADC input, you always get a high voltage regardless of pot position?"
I hope I've explained this a little better. The pots are adjusting the voltage, each pot is sent to a different analogue input on the Ad converter. So I guess the analogue side is working.

'CS has to go low or its in standby.' I'm unsure what this means.

Once again thanks for the help.

[Rob Strand]

The unit is called a Lester k from electro harmonix.

If you are repairing a commercial device then you would expect the micro to be outputting the correct signals.

It's still worth checking the timing since you could be dealing with a fried microcontroller pin.   Typically a pin is stuck either high or low (or even floating but the oscilloscope will pull it down).   You can also get fried inputs on the micro  - all the signals will look OK but the micro still reads junk.

[Digital Larry]

>>SCLK pin
I could see a pulse looking wave on this pin.
15 DOUT Digital data output
Looked like  3.3v on this pin
DOUT heads to spansion IC (memory)
========

If DOUT is just 3.3V then there is no real data coming out.

====

>> 'CS has to go low or its in standby.' I'm unsure what this means.

'CS (chip select, pin 1).  Has to be LOW for the output to be valid.  Normally a Chip Select line is driven low for the entire time that data is coming out.  Since DOUT is just 3.3V solid even though you have what looks like a clock going up and down, I'd suspect something wrong with the 'CS.

[anotherjim]

"/CS" (I don't know how to type an overline) is "Chip Select". The over/bar line means it works at logic Low level. /CS or /CE (chip enable) are almost always present on peripheral devices that can share common bus connections with others. It basically means "You! Yes, I'm talking to you!". The controlling device (MCU) will use at least one of its port pins to give the /CS command or if there are many peripherals, an address decoder chip like a 74HC138 which can provide up to 8 selects from 3 port pins.
In the case of this ADC, the /CS at logic High puts it in low-power "sleep" or "standby" mode. Some other devices might only use the pin to disable/enable their bus connections.

So, have you faulty Lester K or are you tyring to make it into a Lester G?

[9 volts]

I had a quick look last night (flat out lately-even in lockdown!)
CS pin on oscilloscope looks like the same wave as looks as clock.....but inverted!
It would make sense that this ic is in 'sleeping mode'
I'll try to take a photo if possible. Learning curve!
Thanks again for the help, will keep you posted on any progress!

ps
'So, have you faulty Lester K or are you tyring to make it into a Lester G?'
Faulty lester k! speed switch works though!