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From: George Herold on 5 May 2010 12:26
I’m looking at the thermal noise from a resistor down the bottom of a
probe. Cable and probe are maybe 1/2 a meter long. I added a shield
line to try and reduce the capacitance between the ‘active’ end of the
resistor and ground. (The other end of the resistor is tied to ground
at the bottom of the probe.)

The driven shield seemed to work great at the higher impedance levels
1Meg and 100k ohms. (Though a more careful examination showed there
were some issues.) When I tried 10k ohms there was some serious gain
peaking at the higher frequencies...above 100kHz. I mucked about a
bit and made sure this wasn’t the common problem of an opamp driving a
capacitive load.

Late yesterday it struck me that there is capacitive coupling from the
shield back to the input. I had been mistakenly thinking of the
shield as only a capacitance to ground. The capacitance of the inner
conductor to the ground (Cig) is 60pF, from the inner conductor to the
shield (Cis) is 85pF and from the shield to ground (Csg) is 160pF.

I was first using an opamp follower to drive the shield, but later
added a bit of gain...and then threw it away.

+-----Cis--+
| |
| |\ |
+-----+--+ \ |
| | >----+---+----+
| +-- / OPA | | |
Rmeas. | |/ 134 | R1 Csg
Rmeas. | | R1 |
| +---------+ | |
| GND GND
GND

R1 was 50 ohms (to get rid of Csg ringing)
And then this,

+-----Cis--+
| |
| |\ |
+-----+--+ \ |
| | >----+-R4R4--+----+
| +-- / OPA | | |
Rmeas. | |/ 134 | R5 Csg
Rmeas. | | R5 |
| +--R3R3---+ | |
| | GND GND
GND R2
R2
|
GND

Where R3 and R4 were 1kohm and R2 and R5 were 100 ohms.

This seems like it must be a known problem and I wondered if there are
any simple solutions. I thought that a bit of inductance (L =
Cis*Rmeas^2) in the right place might help, but I only managed to make
a nice oscillator.

Thanks for any help or advice,
George H.
From: cassiope on 5 May 2010 12:58
On May 5, 9:26 am, George Herold <ggher...(a)gmail.com> wrote:
> I’m looking at the thermal noise from a resistor down the bottom of a
> probe.  Cable and probe are maybe 1/2 a meter long.  I added a shield
> line to try and reduce the capacitance between the ‘active’ end of the
> resistor and ground.  (The other end of the resistor is tied to ground
> at the bottom of the probe.)
>
> The driven shield seemed to work great at the higher impedance levels
> 1Meg and 100k ohms. (Though a more careful examination showed there
> were some issues.)  When I tried 10k ohms there was some serious gain
> peaking at the higher frequencies...above 100kHz.  I mucked about a
> bit and made sure this wasn’t the common problem of an opamp driving a
> capacitive load.
>
> Late yesterday it struck me that there is capacitive coupling from the
> shield back to the input.  I had been mistakenly thinking of the
> shield as only a capacitance to ground.  The capacitance of the inner
> conductor to the ground (Cig) is 60pF, from the inner conductor to the
> shield (Cis) is 85pF and from the shield to ground (Csg) is 160pF.
>
> I was first using an opamp follower to drive the shield, but later
> added a bit of gain...and then threw it away.
>
>       +-----Cis--+
>       |          |
>       |  |\      |
> +-----+--+ \     |
> |        |  >----+---+----+
> |      +-- / OPA |   |    |
> Rmeas. | |/  134 |   R1   Csg
> Rmeas. |         |   R1   |
> |      +---------+   |    |
> |                   GND   GND
> GND
>
> R1 was 50 ohms (to get rid of Csg ringing)
> And then this,
>
>       +-----Cis--+
>       |          |
>       |  |\      |
> +-----+--+ \     |
> |        |  >----+-R4R4--+----+
> |      +-- / OPA |       |    |
> Rmeas. | |/  134 |       R5   Csg
> Rmeas. |         |       R5   |
> |      +--R3R3---+       |    |
> |      |                 GND  GND
> GND    R2
>        R2
>        |
>        GND
>
> Where R3 and R4 were 1kohm and R2 and R5 were 100 ohms.
>
> This seems like it must be a known problem and I wondered if there are
> any simple solutions.  I thought that a bit of inductance (L =
> Cis*Rmeas^2) in the right place might help, but I only managed to make
> a nice oscillator.
>
> Thanks for any help or advice,
> George H.

All sorts of warning flags should immediately pop up at you when you
connect a capacitor from output to noninverting input.
You might want to look up "negative immitance oscillator".

You may be able to achieve some success with a driven shield, but
you're going to need a much better model of your input,
and tune your circuit to those particulars - at least if you want to
achieve semi-high performance.

HTH...
From: George Herold on 5 May 2010 17:08
On May 5, 12:58 pm, cassiope <f...(a)u.washington.edu> wrote:
> On May 5, 9:26 am, George Herold <ggher...(a)gmail.com> wrote:
>
>
>
>
>
> > I’m looking at the thermal noise from a resistor down the bottom of a
> > probe.  Cable and probe are maybe 1/2 a meter long.  I added a shield
> > line to try and reduce the capacitance between the ‘active’ end of the
> > resistor and ground.  (The other end of the resistor is tied to ground
> > at the bottom of the probe.)
>
> > The driven shield seemed to work great at the higher impedance levels
> > 1Meg and 100k ohms. (Though a more careful examination showed there
> > were some issues.)  When I tried 10k ohms there was some serious gain
> > peaking at the higher frequencies...above 100kHz.  I mucked about a
> > bit and made sure this wasn’t the common problem of an opamp driving a
> > capacitive load.
>
> > Late yesterday it struck me that there is capacitive coupling from the
> > shield back to the input.  I had been mistakenly thinking of the
> > shield as only a capacitance to ground.  The capacitance of the inner
> > conductor to the ground (Cig) is 60pF, from the inner conductor to the
> > shield (Cis) is 85pF and from the shield to ground (Csg) is 160pF.
>
> > I was first using an opamp follower to drive the shield, but later
> > added a bit of gain...and then threw it away.
>
> >       +-----Cis--+
> >       |          |
> >       |  |\      |
> > +-----+--+ \     |
> > |        |  >----+---+----+
> > |      +-- / OPA |   |    |
> > Rmeas. | |/  134 |   R1   Csg
> > Rmeas. |         |   R1   |
> > |      +---------+   |    |
> > |                   GND   GND
> > GND
>
> > R1 was 50 ohms (to get rid of Csg ringing)
> > And then this,
>
> >       +-----Cis--+
> >       |          |
> >       |  |\      |
> > +-----+--+ \     |
> > |        |  >----+-R4R4--+----+
> > |      +-- / OPA |       |    |
> > Rmeas. | |/  134 |       R5   Csg
> > Rmeas. |         |       R5   |
> > |      +--R3R3---+       |    |
> > |      |                 GND  GND
> > GND    R2
> >        R2
> >        |
> >        GND
>
> > Where R3 and R4 were 1kohm and R2 and R5 were 100 ohms.
>
> > This seems like it must be a known problem and I wondered if there are
> > any simple solutions.  I thought that a bit of inductance (L =
> > Cis*Rmeas^2) in the right place might help, but I only managed to make
> > a nice oscillator.
>
> > Thanks for any help or advice,
> > George H.
>
> All sorts of warning flags should immediately pop up at you when you
> connect a capacitor from output to noninverting input.
> You might want to look up "negative immitance oscillator".
>
> You may be able to achieve some success with a driven shield, but
> you're going to need a much better model of your input,
> and tune your circuit to those particulars - at least if you want to
> achieve semi-high performance.
>
> HTH...- Hide quoted text -
>
> - Show quoted text -

Thanks Cassiope, Though I hope you don't think I added the
capacitance. It just came along with the shield. I think I have a
pretty good idea of what the input looks like. (There's not a whole
lot to it.) I did a bit of modeling with LTspice last night, but I
don't know how to model the driven shield. (I just reduced the Cig
capacitance from the starting value of 60pF down to 6pF or
something.) Perhaps the gain peaking is why I've only seen driven
shields on real high impedance inputs. >100Meg

George H.
From: Nemo on 5 May 2010 18:09
The example I've seen (in a Texas app note) had a few ohms in series
with the op amp output.

Surely the effective capacitance from inner conductor to shield, which
you call Cis, is essentially nil because of the driven shield masking
the C - i.e. the inner conductor does not see a potential difference
between itself and the outer conductor.

I'm about to try this myself for a pair of cables about 30cm long. I
chose an AD8626 because -

- the input current is very small
- bandwidth seemed decent, a few MHz
- it can drive a few hundred pF
- it's unity gain stable
- it has low Cin
- it can use +/-15V supplies and drive as near the rails as my signals
are likely to go

I'm connecting the inverting input directly to the output (a voltage
follower) putting a 1 ohm resistor in series with the output, which will
drive the shield.

If you need more info let me know and we can email directly.
--
Nemo
From: John Larkin on 5 May 2010 19:45
On Wed, 5 May 2010 09:26:32 -0700 (PDT), George Herold
<ggherold(a)gmail.com> wrote:

>I�m looking at the thermal noise from a resistor down the bottom of a
>probe. Cable and probe are maybe 1/2 a meter long. I added a shield
>line to try and reduce the capacitance between the �active� end of the
>resistor and ground. (The other end of the resistor is tied to ground
>at the bottom of the probe.)
>
>The driven shield seemed to work great at the higher impedance levels
>1Meg and 100k ohms. (Though a more careful examination showed there
>were some issues.) When I tried 10k ohms there was some serious gain
>peaking at the higher frequencies...above 100kHz. I mucked about a
>bit and made sure this wasn�t the common problem of an opamp driving a
>capacitive load.
>
>Late yesterday it struck me that there is capacitive coupling from the
>shield back to the input. I had been mistakenly thinking of the
>shield as only a capacitance to ground. The capacitance of the inner
>conductor to the ground (Cig) is 60pF, from the inner conductor to the
>shield (Cis) is 85pF and from the shield to ground (Csg) is 160pF.
>
>I was first using an opamp follower to drive the shield, but later
>added a bit of gain...and then threw it away.
>
> +-----Cis--+
> | |
> | |\ |
>+-----+--+ \ |
>| | >----+---+----+
>| +-- / OPA | | |
>Rmeas. | |/ 134 | R1 Csg
>Rmeas. | | R1 |
>| +---------+ | |
>| GND GND
>GND
>
>R1 was 50 ohms (to get rid of Csg ringing)
>And then this,
>
> +-----Cis--+
> | |
> | |\ |
>+-----+--+ \ |
>| | >----+-R4R4--+----+
>| +-- / OPA | | |
>Rmeas. | |/ 134 | R5 Csg
>Rmeas. | | R5 |
>| +--R3R3---+ | |
>| | GND GND
>GND R2
> R2
> |
> GND
>
>Where R3 and R4 were 1kohm and R2 and R5 were 100 ohms.
>
>This seems like it must be a known problem and I wondered if there are
>any simple solutions. I thought that a bit of inductance (L =
>Cis*Rmeas^2) in the right place might help, but I only managed to make
>a nice oscillator.
>
>Thanks for any help or advice,
>George H.

You're trying to measure Johnson noise, right?

One problem with any such guarding/bootstrapping scheme is that there
is a room-temp amplifier that picks up the signal and drives the
guard, and it has noise of its own. In some cases that makes adding
the guard a losing battle.

The OPA134 may be a bit slow, especially driving Csg.

What sort of coax are you using? Can you reduce its capacitance?

John

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