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Part Number: ADC0808-N
I am troubleshooting an issue on a design that uses an ADC0808-N. One observation I have made is that I get different behavior depending on whether I apply voltage to the VCC pin before VREF, or vice versa.
The VCC pin is receiving power from a +5 V supply and is connected directly to the power rail.
The VREF+ pin is connected to +5 V derived from a separate power supply than the VCC +5 V rail. The derived +5 V goes through an op-amp voltage follower (TLV4170) who's output is then connected to VREF+.
The VREF- pin is connected to ground.
When I am powering up the ADC, if I apply voltage to VCC first, and then to VREF+, the ADC works as expected.
However, if I apply voltage to VREF+ first, and then to VCC, I notice that the VREF+ plus pin has dropped from +5 V to about +4.6 V. The output of the voltage follower has dropped to +4.6 V even though its input is still at +5.0 V. It seems like the ADC is in some state where the VREF+ pin is loading down the op-amp.
Anyone know anything about this behavior? Any ideas would be appreciated!
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In reply to Tom Hendrick:
Thanks Tom, I have a gut feeling that the scenario you described is exactly what is happening. You are correct that the ADC still continues to operate even after entering this semi-latch-up state - digital outputs are all off since VREF is not my expected value.
I will experiment and mark this topic closed when I have the data.
In reply to Jason Lo1:
Okay, we have concluded that improper sequencing of VREF and VCC on the ADC was the cause of the error I was seeing. This topic can be marked as resolved. Thanks for the help!
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