Filtering and amplifying a 5mV signal

On Tue, 23 Apr 2019 21:11:36 -0700, John Larkin
<jjlarkin@highlandtechnology.com> wrote:

On Tue, 23 Apr 2019 20:15:33 -0700 (PDT), Fibo <panfilero@gmail.com
wrote:

On Tuesday, April 23, 2019 at 9:16:46 AM UTC-5, John Larkin wrote:
On Tue, 23 Apr 2019 00:51:27 -0700 (PDT), Fibo <fibo@gmail.com
wrote:

On Saturday, April 13, 2019 at 1:38:56 PM UTC-5, Fibo wrote:
Hello,

I was wondering if I could get some advice on how to approach a design. I have a 5mV 8MHz signal I need to feed into an ADC, I want to do 2 things, filter it, and amplify it, the end result needs to fit on a fairly small pcb... something like 2" x 2" , I'm using FilterPro right now to come up with some filter ideas... do you think it's ok to do an active filer that can do the gain and bandpass? Then feed that into an ADC... I'm looking at using an AD9238 (12-bit, 65MSPS) ADC... any tips/thoughts is greatly appreciated!

Much thanks!

Thanks for all the replies, my signal is very sine wave-ish, it's getting lost in noise that is around the same frequency of interest, it's coming off a coax cable, 50ohm source

The trick is to use all the signal power that's available and jam it
into an amplifier. If the signal is 50 ohms and reasonably narrowband,
impedance-match it into a low-noise semiconductor, a bipolar or phemt
or even an opamp. That needs an LC tuned network first thing. You
could easily get a noiseless gain of 5, maybe 10, from a tuned
network.

A mediocre FM or shortwave receiver can play nice music with
microvolts of signal.



I've pulled up the VCVS section in my AoE book, seems pretty do-able, it would be great if I could get a couple stages with G=10

Does using a real fast op-amp increase my chances of getting more high frequency noise? I've never used a CFB amp, I'll check them out

Noise is kinda the main issue we're dealing with, should I consider using a diff-amp to bring in my signal (similar to the order Larkin described) diff-amp then filter then ADC driver?

If the signal comes in on a coax, a diffamp won't help.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

What's a tuned network with gain? I've never done an LC bandpass filter before, but put one together and ran it in LTSpice, and it looks pretty good, would just an LC bandpass filter work for this kind of thing?

What parameters do I need to watch out for on my inductors when doing an LC filter like this?

My LTSpice Filter: https://drive.google.com/drive/u/1/folders/0B57i2560inOAMkNuZnZQalNEVWc

That filter is OK as such, but it loses half of the signal voltage


I don't have a lot of info about the issue, a friend was having an issue reading back data from a sensor and was having to average a bunch of samples to see his signal, seemed like an interesting problem so I've decided to take a crack at it, and learn something along the way

https://drive.google.com/drive/u/1/folders/0B57i2560inOAMkNuZnZQalNEVWc

Much thanks!


It's simple: 50 ohm coax in, series capacitor, inductor to ground,
with a hi-z amplifier at the LC junction. The signal voltage is
amplified by the Q of the LC circuit. Q is limited by your signal
bandwidth requirement and eventually by the inherent Q of the
inductor. Q=10 or maybe 20 is reasonable if your signal is narrowband,
which gives a free, noiseless voltage gain of 10 or 20. The LC
resonant frequency should be tuned to peak the resonance at 8 MHz.

If you need a flat, wider passband, a more complex network can do a
real bandpass function but still step up the impedance and get some
voltage gain. Your waveforms don't look narrowband, so a simple peaked
LC may not be ideal. We don't know anything about the signal.

The amp could be a jfet or a phemt or a fast jfet opamp, something
with just a few pF of capacitance.

If you connect the 50 ohm coax directly into a hi-z amp, even through
a bandpass filter, very little of the available signal power is pushed
into the amp, which is a waste.


Alternately, Mini-circuits makes some 50-ohm matched/untuned/wideband
mmic amps that have low noise figures, below 3 dB at 8 MHz. That's a
noise density below 1 nV/rootHz, which would be <1 uV RMS noise in a 1
MHz bandwidth. That's pretty good for an 8 mV signal. You could put a
bandpass filter *after* the mmic. They are really easy to use. MAR6SM
maybe. 50 ohms straight in.

https://www.minicircuits.com/pdfs/MAR-6SM+.pdf

Really, 8 mV is a gigantic signal.
Click to expand...

Actually, with 50 ohms on both ends, there's no penalty for putting an
LC bandpass before a MMIC. That would reduce nonlinearity overload
possibilities.




--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
 
On Wednesday, April 24, 2019 at 11:58:36 AM UTC-5, John Larkin wrote:
On Tue, 23 Apr 2019 21:11:36 -0700, John Larkin

On Tue, 23 Apr 2019 20:15:33 -0700 (PDT), Fibo <fibo@gmail.com
wrote:

On Tuesday, April 23, 2019 at 9:16:46 AM UTC-5, John Larkin wrote:
On Tue, 23 Apr 2019 00:51:27 -0700 (PDT), Fibo <fibo@gmail.com
wrote:

On Saturday, April 13, 2019 at 1:38:56 PM UTC-5, Fibo wrote:
Hello,

I was wondering if I could get some advice on how to approach a design. I have a 5mV 8MHz signal I need to feed into an ADC, I want to do 2 things, filter it, and amplify it, the end result needs to fit on a fairly small pcb... something like 2" x 2" , I'm using FilterPro right now to come up with some filter ideas... do you think it's ok to do an active filer that can do the gain and bandpass? Then feed that into an ADC... I'm looking at using an AD9238 (12-bit, 65MSPS) ADC... any tips/thoughts is greatly appreciated!

Much thanks!

Thanks for all the replies, my signal is very sine wave-ish, it's getting lost in noise that is around the same frequency of interest, it's coming off a coax cable, 50ohm source

The trick is to use all the signal power that's available and jam it
into an amplifier. If the signal is 50 ohms and reasonably narrowband,
impedance-match it into a low-noise semiconductor, a bipolar or phemt
or even an opamp. That needs an LC tuned network first thing. You
could easily get a noiseless gain of 5, maybe 10, from a tuned
network.

A mediocre FM or shortwave receiver can play nice music with
microvolts of signal.



I've pulled up the VCVS section in my AoE book, seems pretty do-able, it would be great if I could get a couple stages with G=10

Does using a real fast op-amp increase my chances of getting more high frequency noise? I've never used a CFB amp, I'll check them out

Noise is kinda the main issue we're dealing with, should I consider using a diff-amp to bring in my signal (similar to the order Larkin described) diff-amp then filter then ADC driver?

If the signal comes in on a coax, a diffamp won't help.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

What's a tuned network with gain? I've never done an LC bandpass filter before, but put one together and ran it in LTSpice, and it looks pretty good, would just an LC bandpass filter work for this kind of thing?

What parameters do I need to watch out for on my inductors when doing an LC filter like this?

My LTSpice Filter: https://drive.google.com/drive/u/1/folders/0B57i2560inOAMkNuZnZQalNEVWc

That filter is OK as such, but it loses half of the signal voltage


I don't have a lot of info about the issue, a friend was having an issue reading back data from a sensor and was having to average a bunch of samples to see his signal, seemed like an interesting problem so I've decided to take a crack at it, and learn something along the way

https://drive.google.com/drive/u/1/folders/0B57i2560inOAMkNuZnZQalNEVWc

Much thanks!


It's simple: 50 ohm coax in, series capacitor, inductor to ground,
with a hi-z amplifier at the LC junction. The signal voltage is
amplified by the Q of the LC circuit. Q is limited by your signal
bandwidth requirement and eventually by the inherent Q of the
inductor. Q=10 or maybe 20 is reasonable if your signal is narrowband,
which gives a free, noiseless voltage gain of 10 or 20. The LC
resonant frequency should be tuned to peak the resonance at 8 MHz.

If you need a flat, wider passband, a more complex network can do a
real bandpass function but still step up the impedance and get some
voltage gain. Your waveforms don't look narrowband, so a simple peaked
LC may not be ideal. We don't know anything about the signal.

The amp could be a jfet or a phemt or a fast jfet opamp, something
with just a few pF of capacitance.

If you connect the 50 ohm coax directly into a hi-z amp, even through
a bandpass filter, very little of the available signal power is pushed
into the amp, which is a waste.


Alternately, Mini-circuits makes some 50-ohm matched/untuned/wideband
mmic amps that have low noise figures, below 3 dB at 8 MHz. That's a
noise density below 1 nV/rootHz, which would be <1 uV RMS noise in a 1
MHz bandwidth. That's pretty good for an 8 mV signal. You could put a
bandpass filter *after* the mmic. They are really easy to use. MAR6SM
maybe. 50 ohms straight in.

https://www.minicircuits.com/pdfs/MAR-6SM+.pdf

Really, 8 mV is a gigantic signal.

Actually, with 50 ohms on both ends, there's no penalty for putting an
LC bandpass before a MMIC. That would reduce nonlinearity overload
possibilities.




--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
Click to expand...

thanks! I think after chasing various solutions, and struggling with simulating active gain/filter stages at this frequency, I'm liking the passive LC route, also looking into that MMIC

would an LC filter with a discrete transistor gain stage of been better than opamps?
 
On Wed, 24 Apr 2019 11:08:35 -0700 (PDT), Fibo <panfilero@gmail.com>
wrote:

On Wednesday, April 24, 2019 at 11:58:36 AM UTC-5, John Larkin wrote:
On Tue, 23 Apr 2019 21:11:36 -0700, John Larkin

On Tue, 23 Apr 2019 20:15:33 -0700 (PDT), Fibo <fibo@gmail.com
wrote:

On Tuesday, April 23, 2019 at 9:16:46 AM UTC-5, John Larkin wrote:
On Tue, 23 Apr 2019 00:51:27 -0700 (PDT), Fibo <fibo@gmail.com
wrote:

On Saturday, April 13, 2019 at 1:38:56 PM UTC-5, Fibo wrote:
Hello,

I was wondering if I could get some advice on how to approach a design. I have a 5mV 8MHz signal I need to feed into an ADC, I want to do 2 things, filter it, and amplify it, the end result needs to fit on a fairly small pcb... something like 2" x 2" , I'm using FilterPro right now to come up with some filter ideas... do you think it's ok to do an active filer that can do the gain and bandpass? Then feed that into an ADC... I'm looking at using an AD9238 (12-bit, 65MSPS) ADC... any tips/thoughts is greatly appreciated!

Much thanks!

Thanks for all the replies, my signal is very sine wave-ish, it's getting lost in noise that is around the same frequency of interest, it's coming off a coax cable, 50ohm source

The trick is to use all the signal power that's available and jam it
into an amplifier. If the signal is 50 ohms and reasonably narrowband,
impedance-match it into a low-noise semiconductor, a bipolar or phemt
or even an opamp. That needs an LC tuned network first thing. You
could easily get a noiseless gain of 5, maybe 10, from a tuned
network.

A mediocre FM or shortwave receiver can play nice music with
microvolts of signal.



I've pulled up the VCVS section in my AoE book, seems pretty do-able, it would be great if I could get a couple stages with G=10

Does using a real fast op-amp increase my chances of getting more high frequency noise? I've never used a CFB amp, I'll check them out

Noise is kinda the main issue we're dealing with, should I consider using a diff-amp to bring in my signal (similar to the order Larkin described) diff-amp then filter then ADC driver?

If the signal comes in on a coax, a diffamp won't help.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

What's a tuned network with gain? I've never done an LC bandpass filter before, but put one together and ran it in LTSpice, and it looks pretty good, would just an LC bandpass filter work for this kind of thing?

What parameters do I need to watch out for on my inductors when doing an LC filter like this?

My LTSpice Filter: https://drive.google.com/drive/u/1/folders/0B57i2560inOAMkNuZnZQalNEVWc

That filter is OK as such, but it loses half of the signal voltage


I don't have a lot of info about the issue, a friend was having an issue reading back data from a sensor and was having to average a bunch of samples to see his signal, seemed like an interesting problem so I've decided to take a crack at it, and learn something along the way

https://drive.google.com/drive/u/1/folders/0B57i2560inOAMkNuZnZQalNEVWc

Much thanks!


It's simple: 50 ohm coax in, series capacitor, inductor to ground,
with a hi-z amplifier at the LC junction. The signal voltage is
amplified by the Q of the LC circuit. Q is limited by your signal
bandwidth requirement and eventually by the inherent Q of the
inductor. Q=10 or maybe 20 is reasonable if your signal is narrowband,
which gives a free, noiseless voltage gain of 10 or 20. The LC
resonant frequency should be tuned to peak the resonance at 8 MHz.

If you need a flat, wider passband, a more complex network can do a
real bandpass function but still step up the impedance and get some
voltage gain. Your waveforms don't look narrowband, so a simple peaked
LC may not be ideal. We don't know anything about the signal.

The amp could be a jfet or a phemt or a fast jfet opamp, something
with just a few pF of capacitance.

If you connect the 50 ohm coax directly into a hi-z amp, even through
a bandpass filter, very little of the available signal power is pushed
into the amp, which is a waste.


Alternately, Mini-circuits makes some 50-ohm matched/untuned/wideband
mmic amps that have low noise figures, below 3 dB at 8 MHz. That's a
noise density below 1 nV/rootHz, which would be <1 uV RMS noise in a 1
MHz bandwidth. That's pretty good for an 8 mV signal. You could put a
bandpass filter *after* the mmic. They are really easy to use. MAR6SM
maybe. 50 ohms straight in.

https://www.minicircuits.com/pdfs/MAR-6SM+.pdf

Really, 8 mV is a gigantic signal.

Actually, with 50 ohms on both ends, there's no penalty for putting an
LC bandpass before a MMIC. That would reduce nonlinearity overload
possibilities.




--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com


thanks! I think after chasing various solutions, and struggling with simulating active gain/filter stages at this frequency, I'm liking the passive LC route, also looking into that MMIC

would an LC filter with a discrete transistor gain stage of been better than opamps?
Click to expand...

The best noise performance would result from an optimized
matching/filtering network and some discrete fet, but the MMIC would
sure be easy. Looks like a 3dB NF MMIC would be great with your 8 mv
signal, with a bit of filtering on the output side to kill out-of-band
noise.

These Darlington MMICS are cheap and simple and stable and they just
always work. AC coupled of course. We've used them in pulse
applications too, with a little cheating on bias. The ERA series is
good too.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
 
On Tuesday, April 23, 2019 at 8:15:38 PM UTC-7, Fibo wrote:

I was wondering if I could get some advice on how to approach a design. I have a 5mV 8MHz signal I need to feed into an ADC,
Click to expand...

I don't have a lot of info about the issue, a friend was having an issue reading back data from a sensor and was having to average a bunch of samples to see his signal, seemed like an interesting problem so I've decided to take a crack at it, and learn something along the way

https://drive.google.com/drive/u/1/folders/0B57i2560inOAMkNuZnZQalNEVWc
Click to expand...

That shows a sequence of bursts, maybe half dozen cycles each. The most appropriate
filter for such things is a wavelet transform, and if you can 'add' the signals, it means
that the times are synchronized somehow (which means you can constrain to
a limited number of degrees of freedom, which is good).

I suspect a combination of summing and maybe autocorrelating (another kind of summing,
that adds burst #N to burst #N + M) can get you amplitudes, phases, and repeat times. Which
If any of those are the useful data, it's hard to say, but clean signals might be extracted without
too much use of creativity or wishful thinking.
 
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