On Wednesday, January 9, 2019 at 8:09:07 AM UTC-5, Phil Hobbs wrote:
On 1/8/19 7:56 PM, George Herold wrote:
On Tuesday, January 8, 2019 at 5:10:44 PM UTC-5, John Larkin wrote:
On Tue, 8 Jan 2019 10:03:59 -0500, Phil Hobbs
On 1/8/19 8:51 AM, George Herold wrote:
On Monday, January 7, 2019 at 8:44:16 PM UTC-5, Phil Hobbs wrote:
On 1/7/19 8:27 PM, George Herold wrote:
On Monday, January 7, 2019 at 8:01:28 PM UTC-5, bitrex wrote:
On 01/07/2019 07:52 PM, George Herold wrote:
On Monday, January 7, 2019 at 7:37:48 PM UTC-5, bitrex wrote:
On 01/07/2019 07:23 PM, George Herold wrote:
On Monday, January 7, 2019 at 2:34:58 PM UTC-5, Phil Allison wrote:
George Herold wrote:
Perhaps a silly simple question. I need to subtract off a 2.5V
background from a few mV signal. I first think about a difference amp,
or instrument amp.
But I wondered if there is any advantage to a simple
opamp summing amp. (I'll add in -2.5 V)
** Considered using a capacitor ??
Like a sample and hold? It needs to hold the offset
sorta forever, so a pot called for.
DC blocking capacitor. Y'know, like a high-pass on the input.
Oh no we are talking slowly varying signals. Minutes to
an hour maybe..
So it's my bosses idea, you put a hall probe sensor inside
a High Tc super conductor toroid. Induce a super current.
(by cooling down with permanant magnet in the toroid.)
And then observe the field go away as you warm it up.
Got it, I figured you had not forgotten how to use a capacitor to block
DC but just so long as we cover all the bases here and the signal period
wasn't mentioned explicitly ;)
Oh dear no, big C, big R and fet opamp.. about 1 second is the longest
TC I can wait for. (10uF and 100k or 1uF and a meg.)
I was doing that today to look at power supply noise.
I think most of the circuits I build are 'scope preamps. :^)
I've never done the feedback cap thing, (you posted previoulsy),
looks like a Sallen-Key.
If you want to amplify the signal by N times and the offset by 1x (so
that you're still biased above ground), that's easy to do accurately
with a garden-variety noninverting amp.
I don't really care where the signal level is, I just want to look at
(small) changes. I'm trying to picture how a non-iverting amp
does this, but can't. Can you give me more of a hint?
A noninverting gain of N amplifier is also an inverting gain of N-1
amplifier, and the 1 is very accurate when the loop gain is high. Thus
if you have an offset of 2.50000V, connecting the input resistor of the
op amp (on the inverting side) to your offset generator and the
noninverting input to signal+offset, you get
Vout = (N+1)(Vos + Vsig) - N Vos = Vos + N Vsig.
This is just like the usual split-supply, single-ended noninverting amp,
except with the ground reference relabelled. Resistor inaccuracy causes
gain inaccuracy, but the offset remains correct.
This move is super common in single-supply data acq circuits, for instance.
That's a nice concept: get some single-ended gain first, then take out
Huh, OK. It's probably that I'm a circuit wusse, but inverting gain
w/o a ground reference bothers me. I can try it. One could HP
filter the output.
The gain is all noninverting. At the output, both signal and offset get
multiplied by positive numbers. The numbers are different, is all.
There's no difference whatsoever between this and a normal split supply
noninverting stage except that a different node is labelled 'ground'.
Right... my comment was a little tongue in cheek.. I should have added
the smiley face.
(In my case)
It looks like there is so much 1/f noise in the sensor that nothing
else maters too much. (Lasse's back to back seems to work fine,
'in theory' with equal noise 'densities' I should get a sqrt(2)
improvement in S/N. That has to apply to 1/f noise too? No?
If the origins of the noise are independent, sure. Air currents or
power supply junk, no.
In one of my early consulting gigs, I was building stabilized lasers for
laser gravimeters down in Albuquerque. They had to be very stable over
periods of hours and days. I had a brass plaque made for my cube wall
The Final Frontier
Yeah it's a little hard to measure the lower frequencies of 1/f noise,
things below 1 Hz.
Not hard, just tedious. Data loggers make it easier, but you do have to accumulate data for minutes or longer.