HELP, I have non-linear gain in two channels

[Elektro]

I have two identical 3 stage active band pass filters consisting of 3
op amps each. The problem is that I get varying gain in the channels
depending on the input signal level.

For a test I ran the same signal to both channels at the same time and
sample the output from each channel. Those sampled signal are then
adjusted so the static gain is the same for both channels. And then
the bias is adjusted.

If I then plot the signals they seem to be exactly the same. But if I
plot the deferens between the channels I can se the sometimes ch1 is a
bit more than ch2 and vise versa. It's dependent of the input signal
amplitude.

What can cause this, any tips?

In my application it is important that both channels change the same
percentage. If the input signal to ch1 and ch2 attenuates 1% the
outputs from ch1 and ch2 must attenuate 1% also.

ADC's can have small non-linearity errors. So maybe I'm going to
connect one band pass signal to both ADC's to check if I get the same
result.

 

[Martin Griffith]

I have two identical 3 stage active band pass filters consisting of 3
op amps each. The problem is that I get varying gain in the channels
depending on the input signal level.

For a test I ran the same signal to both channels at the same time and
sample the output from each channel. Those sampled signal are then
adjusted so the static gain is the same for both channels. And then
the bias is adjusted.

If I then plot the signals they seem to be exactly the same. But if I
plot the deferens between the channels I can se the sometimes ch1 is a
bit more than ch2 and vise versa. It's dependent of the input signal
amplitude.

What can cause this, any tips?

In my application it is important that both channels change the same
percentage. If the input signal to ch1 and ch2 attenuates 1% the
outputs from ch1 and ch2 must attenuate 1% also.

ADC's can have small non-linearity errors. So maybe I'm going to
connect one band pass signal to both ADC's to check if I get the same
result.

What's the Q of the stages? What type of bpf? What frequency ranges,
DC to 1Hz, or DC to 0.5C +- 3dB


Martin

 

[Jim Thompson]

I have two identical 3 stage active band pass filters consisting of 3
op amps each. The problem is that I get varying gain in the channels
depending on the input signal level.

For a test I ran the same signal to both channels at the same time and
sample the output from each channel. Those sampled signal are then
adjusted so the static gain is the same for both channels. And then
the bias is adjusted.

If I then plot the signals they seem to be exactly the same. But if I
plot the deferens between the channels I can se the sometimes ch1 is a
bit more than ch2 and vise versa. It's dependent of the input signal
amplitude.

What can cause this, any tips?

In my application it is important that both channels change the same
percentage. If the input signal to ch1 and ch2 attenuates 1% the
outputs from ch1 and ch2 must attenuate 1% also.

ADC's can have small non-linearity errors. So maybe I'm going to
connect one band pass signal to both ADC's to check if I get the same
result.

Poke around with an oscilloscope. One of the stages is clipping...
typical for band-pass filters (may have internal node gains
approaching "Q", depending on filter architecture).

...Jim Thompson

 

[Vladimir Vassilevsky]

I have two identical 3 stage active band pass filters consisting of 3
op amps each. The problem is that I get varying gain in the channels
depending on the input signal level.

How big is the variation?

For a test I ran the same signal to both channels at the same time and
sample the output from each channel. Those sampled signal are then
adjusted so the static gain is the same for both channels. And then
the bias is adjusted.

If it is bandpass, then it is AC. If it is AC, why bias does matter?

If I then plot the signals they seem to be exactly the same. But if I
plot the deferens between the channels I can se the sometimes ch1 is a
bit more than ch2 and vise versa. It's dependent of the input signal
amplitude.

What can cause this, any tips?

Before searching into the complicated problems, I would check for the
simple stuff. It could be the power frequency or some other noise picked
up by the channels in the different proportion. It could also be the
effect of cross coupling between the channels. It could be the bad
voltage references for the ADCs.

Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

 

[john jardine]

I have two identical 3 stage active band pass filters consisting of 3
op amps each. The problem is that I get varying gain in the channels
depending on the input signal level.

For a test I ran the same signal to both channels at the same time and
sample the output from each channel. Those sampled signal are then
adjusted so the static gain is the same for both channels. And then
the bias is adjusted.

If I then plot the signals they seem to be exactly the same. But if I
plot the deferens between the channels I can se the sometimes ch1 is a
bit more than ch2 and vise versa. It's dependent of the input signal
amplitude.

What can cause this, any tips?

In my application it is important that both channels change the same
percentage. If the input signal to ch1 and ch2 attenuates 1% the
outputs from ch1 and ch2 must attenuate 1% also.

ADC's can have small non-linearity errors. So maybe I'm going to
connect one band pass signal to both ADC's to check if I get the same
result.

I wouldn't have thought it possible that 2 channels of 9 opamps and shed
loads of passive components are ever going to match within 1%.
If you're plotting the deferens on a cycle by cycle basis then tiny phase
shifts will give big errors. Even with an RMS calculation for each channel
there will be differences, due to the numerous capacitors changing their
values slightly with differing signal levels.
God forbid you using anything remotely like ceramic caps. These can offer a
(useful?) 2X value change for a 2X signal change.

 

[crominator]

I have two identical 3 stage active band pass filters consisting of 3
op amps each. The problem is that I get varying gain in the channels
depending on the input signal level.

For a test I ran the same signal to both channels at the same time and
sample the output from each channel. Those sampled signal are then
adjusted so the static gain is the same for both channels. And then
the bias is adjusted.

If I then plot the signals they seem to be exactly the same. But if I
plot the deferens between the channels I can se the sometimes ch1 is a
bit more than ch2 and vise versa. It's dependent of the input signal
amplitude.

What can cause this, any tips?

In my application it is important that both channels change the same
percentage. If the input signal to ch1 and ch2 attenuates 1% the
outputs from ch1 and ch2 must attenuate 1% also.

ADC's can have small non-linearity errors. So maybe I'm going to
connect one band pass signal to both ADC's to check if I get the same
result.

I agree with Jim, you are likely seeing the effects of clipping....
another possibility, see what the GBW product is on the op-amps
you're using and if it is sufficient for the application.

darn Q's always get in the way,
Crom

 

[Elektro]

I have two identical 3 stage active band pass filters consisting of 3
op amps each. The problem is that I get varying gain in the channels
depending on the input signal level.

For a test I ran the same signal to both channels at the same time and
sample the output from each channel. Those sampled signal are then
adjusted so the static gain is the same for both channels. And then
the bias is adjusted.

If I then plot the signals they seem to be exactly the same. But if I
plot the deferens between the channels I can se the sometimes ch1 is a
bit more than ch2 and vise versa. It's dependent of the input signal
amplitude.

What can cause this, any tips?

In my application it is important that both channels change the same
percentage. If the input signal to ch1 and ch2 attenuates 1% the
outputs from ch1 and ch2 must attenuate 1% also.

ADC's can have small non-linearity errors. So maybe I'm going to
connect one band pass signal to both ADC's to check if I get the same
result.

More info

Hello again

I ran a test today. I connected the signal from one of the band pass
channels to both AD converters so they receive the same signal. The
gain difference was almost totally gone. I hade only a small gain
difference which I think is the ADCs linearity error. The dynamic gain
is acceptable.

So the dynamic gain "fault" is located to the band pass filters.

And, yes, I use ceramic capacitors.

 

[Elektro]

I agree with Jim, you are likely seeing the effects of clipping....
another possibility, see what the GBW product is on the op-amps
you're using and if it is sufficient for the application.

darn Q's always get in the way,
Crom- Dölj citerad text -

- Visa citerad text -

I can't se any clipping.

 

[Jim Thompson]

I can't se any clipping.

It may not be readily apparent in the output. Check internal nodes.

...Jim Thompson

 

[john jardine]

[...]

So the dynamic gain "fault" is located to the band pass filters.

And, yes, I use ceramic capacitors.

(just my luck for you to have used hand selected, non commercial, NPO types
in a computer optimised R.F. BP arrangement, running at 1V5, so ignore the
following. Others may find it of passing interest

'Low K','COG', 'NP0', ceramics are nice but only run a few hundred pF and
can be fine for many R.F uses.
Leaves the 'medium K' and 'high K' types which get worse all the way up to
about 100nF.
I've seen a lot of talk here but no hard numbers, so stuck a 100nF (worst
case, high K) 100nF across a variable DC supply and measured it.
Problems immediately turned up as I couldn't finger a 100nF cap'. They were
all showing 90nF. Turns out they are "100nF" but sunshine (rare in UK) was
warming the room up to 24degC and the appalling tempco of these things had
caused the value shift. (body heat from 2 fingers dropped C value to 65nF!).
DC bias voltage C value (at 24degC)
0v 90nF
1V 89nF
2V 86nF
4V 78nF
8V 61nF
16V 39nF
30V 18nF (ie 18% of marked value!)
(The 'bias' can be static DC or the signal itself. )

Cap' was so p*** poor, that for confirmation I knocked up this L.F. voltage
controlled oscillator.
___
,---|___|---,
| 22k |
| |
| |
| 1|\ 2 |
o----| >O---o-----o Frequency Out
| |/
| CD40106 (hex schmidtt)
| (12V supply)
---
--- 470nF
100k |
___ |
0-12V o-----|___|----o
DC Control |
|
---
o ---
| | 100nF ceramic
=== ===
0V 0V
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

Control V Frequency
0 840Hz
2V 843Hz
4V 920Hz
6V 1029Hz
8V 1152Hz
10V 1295Hz
12V 1443Hz
(30V 2840Hz)

(As a final insult, the cap' ESR measured 47ohms at 1kHz, suggesting any
filter resistors need to be >4k7).
Basically for filter use, I'd be incined to use -anything- other than
ceramics

I still love ceramic caps though. They're small, cheap and are excellent for
decoupling use.

 

[Elektro]

[...]

So the dynamic gain "fault" is located to the band pass filters.

And, yes, I use ceramic capacitors.

(just my luck for you to have used hand selected, non commercial, NPO types
in a computer optimised R.F. BP arrangement, running at 1V5, so ignore the
following. Others may find it of passing interest

'Low K','COG', 'NP0', ceramics are nice but only run a few hundred pF and
can be fine for many R.F uses.
Leaves the 'medium K' and 'high K' types which get worse all the way up to
about 100nF.
I've seen a lot of talk here but no hard numbers, so stuck a 100nF (worst
case, high K) 100nF across a variable DC supply and measured it.
Problems immediately turned up as I couldn't finger a 100nF cap'. They were
all showing 90nF. Turns out they are "100nF" but sunshine (rare in UK) was
warming the room up to 24degC and the appalling tempco of these things had
caused the value shift. (body heat from 2 fingers dropped C value to 65nF!).
DC bias voltage C value (at 24degC)
0v 90nF
1V 89nF
2V 86nF
4V 78nF
8V 61nF
16V 39nF
30V 18nF (ie 18% of marked value!)
(The 'bias' can be static DC or the signal itself. )

Cap' was so p*** poor, that for confirmation I knocked up this L.F. voltage
controlled oscillator.
___
,---|___|---,
| 22k |
| |
| |
| 1|\ 2 |
o----| >O---o-----o Frequency Out
| |/
| CD40106 (hex schmidtt)
| (12V supply)
---
--- 470nF
100k |
___ |
0-12V o-----|___|----o
DC Control |
|
---
o ---
| | 100nF ceramic
=== ===
0V 0V
(created by AACircuit v1.28 beta 10/06/04www.tech-chat.de)

Control V Frequency
0 840Hz
2V 843Hz
4V 920Hz
6V 1029Hz
8V 1152Hz
10V 1295Hz
12V 1443Hz
(30V 2840Hz)

(As a final insult, the cap' ESR measured 47ohms at 1kHz, suggesting any
filter resistors need to be >4k7).
Basically for filter use, I'd be incined to use -anything- other than
ceramics

I still love ceramic caps though. They're small, cheap and are excellent for
decoupling use.

Thank youy for your interesting reply I will try to change to
other kind of caps.