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Weird Attenuator Problem
Weird Attenuator Problem
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Old 25th February 2020, 06:58 PM   #1
simonra is offline simonra  United Kingdom
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Default Weird Attenuator Problem

I've been working on a preamp design with a log stepped attenuator and I've come across a strange problem. Overall performance is good except when it's set to -0.5dB or -32 dB (when the stages are set 1000000 or 0000001).
When I started to probe the circuit I found that the distortion disappeared as soon as the probe touched the signal circuit.

Any ideas what's going on? It happens even when I have the probe BNC disconnected.

The attenuator is 500Ω with 7 stages (1/2, 1, 2, 4, 8, 16, 32dB) with an OPA1612 buffering the input. I originally thought that I might be overloading the buffer op amp but the result is the same with lower input signal voltages.

Ideas appreciated.
Thanks.
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File Type: png Without Probe.png (446.4 KB, 146 views)
File Type: png With Probe.png (452.6 KB, 140 views)
File Type: png schematic.png (102.9 KB, 149 views)
File Type: png layout.png (90.6 KB, 151 views)
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Old 25th February 2020, 07:03 PM   #2
Mooly is offline Mooly  United Kingdom
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Weird Attenuator Problem
A quick thought would be the capacitance of the probe is killing some extreme HF instability somewhere. A finger judiciously applied to various areas might get a handle on it.
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Old 25th February 2020, 07:07 PM   #3
simonra is offline simonra  United Kingdom
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I thought the same... I've tried poking around and putting a 10pF across the same points but nothing.
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Old 25th February 2020, 07:18 PM   #4
Mooly is offline Mooly  United Kingdom
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Weird Attenuator Problem
I wonder if the long traces are causing a problem somehow. A small series resistor at the opamp output... well you know how it goes

Might be worth trying playing around with the noise gain of the opamps by adding a resistor and series cap between the two inputs.. Begin with just the resistor, it won't alter the signal gain.

Its a case of getting an idea where in the chain the problem begins, at least to begin with.
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Old 25th February 2020, 07:25 PM   #5
simonra is offline simonra  United Kingdom
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Good plan... the traces are quite long and I should have put 50Ω on the op amp output. I'll see if I can find a way of inserting one in the circuit. I've become so reliant on probing the circuit to fault-find this one's thrown me because the fault disappears the second the probe makes contact.
Thanks.
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Old 25th February 2020, 07:25 PM   #6
rayma is offline rayma  United States
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Try the circuit with only one op amp at a time, to see if just one of them is causing the problem.
They do each have bypass capacitors? Does the input op amp actually have 27 ohm feedback resistors?
That is way too low for the first op amp's output, plus there's the resistor network loading in parallel.

Last edited by rayma; 25th February 2020 at 07:43 PM.
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Old 25th February 2020, 07:29 PM   #7
Mooly is offline Mooly  United Kingdom
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Weird Attenuator Problem
Good luck I'll look in tomorrow and see if its fixed
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Old 25th February 2020, 08:05 PM   #8
simonra is offline simonra  United Kingdom
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Quote:
Originally Posted by rayma View Post
Try the circuit with only one op amp at a time, to see if just one of them is causing the problem.
They do each have bypass capacitors? Does the input op amp actually have 27 ohm feedback resistors?
That is way too low for the first op amp's output, plus there's the resistor network loading in parallel.
Hah, no sorry I quickly put together the circuit in Tina TI and copied R1 without changing the value, R16 & 17 are 1k. 100nF x 2 on both op amps as close as I can get them to the supply pins.

I'm up for taking the output op amp out of circuit but wouldn't mind trying a few less invasive things first.
Thanks.
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Old 25th February 2020, 08:07 PM   #9
rayma is offline rayma  United States
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Does the distortion show up at the output of the resistor network?
How about at the output of the first op amp?

Last edited by rayma; 25th February 2020 at 08:10 PM.
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Old 25th February 2020, 08:47 PM   #10
sgrossklass is offline sgrossklass  Germany
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I reckon you need some resistance in series with OP2's input. 50-100 ohms tops. Clearly the problem occurs when source impedance at this point is particularly low. It's a problem particularly afflicting low-noise BJT inputs. BJTs do not like predominantly inductive source impedance, their beta(f) curve tends to turn it into negative impedance at the emitter.

BTW, are you sure about the 27 ohm resistors in OP1's feedback network? Is that an AD797 class part, at rather low levels (maybe 1 Vrms tops)? This would be the only constellation where these would make sense to me. Opamps certainly aren't built to drive loads like that at any kind of substantial level (use a buffer and multiple resistors for super wide dynamic range applications). With values this low, some attention to shared ground resistance may be needed as well.
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