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ADC0820-N: oscillating outputs

Ryan Sarver1
Prodigy 190 points
Part Number: ADC0820-N

Hi,

I'm trying to use the ADC0820 in an application without a microcontroller, so free-running. I'm not sure if I connected it properly to use it in that way; it seems to be working partially, but I'm getting oscillation on the output pins specifically at certain input voltages. It's not just during a couple mV or something where it transitions between states (although I am seeing that as well), but rather I get for example a range of about 250mV in VIN that oscillates.

Here's my schematic for reference:

As you can see, I'm not using the two LSBs. The other outputs go to the jam inputs of a CD4059 counter. The ADC is powered from an 8V linear regulator (TL750L08CLP).

I only have a few discrete VINs of interest, because I'm just looking to switch between a few output patterns to control the counter.

So for example, one of those output patterns I'm looking to get is '100000' (that's in binary - again, not using the two LSBs). If my math's right, I've got a resolution of 31mV per count with this 8-bit ADC and a reference voltage of 7.96V. So since I'm not using the two LSBs, I should actually be fine to have a result of anything from 128 to 131 counts, corresponding to a VIN range of 3.98 - 4.07V.

In my final application I'm hoping to use a different control scheme but for now I'm just testing with a 10k trim pot between VREF+/VREF-, with the wiper going to VIN.

What I'm seeing in practice, looking at DB2 for example, is that I get a constant 1 at 3.85/3.86V, then from about 3.87V all the way up to 4.10V I get oscillation between 0 and 1 at about 459kHz. Then it settles back to a constant 1 again. So unfortunately that oscillation is happening right where I want the VIN to be.

Here's a scope capture (Yellow is VIN, Purple is DB2):

You can see that the oscillation also imposes noise on the input. That noise is not there when the outputs aren't oscillating:

As a final note, these oscillations are happening on all of the outputs, not just DB2. Many times they seem to start at the same VIN, but sometimes not.

Any help would be much appreciated!

thanks,

Ryan

  • 0
    TI__Guru**** 168605 points

    Hi Ryan,

    I suspect the change in D2 is not an 'oscillation' per se, but the bit actually toggling for some reason - can you put another channel of your scope on the INT/RD line and see what it's doing?  I suspect that 456kHz is the related to the conversion time of the ADC0820 and you'll see that reflected in the INT output.  I've never used the ADC0820 this way so give me a little time to refresh my memory with the datasheet.  I'll get back to you by COB Friday. 

  • 0 in reply to Tom Hendrick
    Prodigy 190 points

    Hi Tom,

    Thanks for your quick response.

    Well the INT/RD node is toggling at 2x the frequency of the toggling of the outputs, so 906kHz (with the outputs toggling at 453kHz).

    But, in doing that test, I think that actually revealed a solution! When I applied the scope probe, the output toggling went away. I assumed it was related to the capacitance of the probe, so I added a 100pF cap from INT/RD to GND, and now it's working as expected!

    Any insight into what might be going on there? Is the cap a good, i.e. reliable fix?

    thanks,

    Ryan

  • 0 in reply to Ryan Sarver1
    TI__Guru**** 168605 points

    Hi Ryan,

    Can you provide the other control signals through a scope capture for your setup?  Everything I see so far has the WR/RDY input controlling the data access with the RD pin tied low.  I'm having a bit of trouble finding examples on exactly how to control the ADC0820 in 'stand alone' mode and I don't have any lab access at the moment to look into this directly, so seeing your other control lines would help!

  • 0 in reply to Tom Hendrick
    Prodigy 190 points

    Hi Tom,

    What you see in the schematic there is all there is regarding the control lines. WR/RDY is left floating. RD/INT are tied together and toggling at 906kHz. OFL's open; CS and MODE are tied to GND.

  • 0 in reply to Ryan Sarver1
    Prodigy 190 points

    Just an extra bit of info I discovered today: I was testing with several different input voltages (having added the 100pF cap to stabilize the output toggling). Everything was working fine at first, but then I noticed a few specific input voltages that just weren't giving the right output for some reason. I increased the 100pF to 1nF and now everything's working as expected as far as I can tell.

  • 0 in reply to Ryan Sarver1
    TI__Guru**** 168605 points

    Great news Ryan!  Happy to hear that your solution is working!