PIN photometer: double opamp

[Dominic-Luc Webb]

Latest adventure is a minimum component PIN
photometer. Sensitivity is an issue (measuring
from faint stars on telescope). I am thinking
to try two opamps to increase weak signals.
Perhaps more amplification is self defeating,
since this is not a PMT. For feedback, I chose
an 0.82 pF capacitor rather than 10 GOhm resistor
as these are very hard to find and expensive. I
think this circuit should work, but would be very
grateful for suggestions for improvements. Please
consider this is a light meter so it should have
a linear response to photon flux.

Dominic

 

[Winfield Hill]

Dominic-Luc Webb wrote...

Latest adventure is a minimum component PIN
photometer. Sensitivity is an issue (measuring
from faint stars on telescope). I am thinking
to try two opamps to increase weak signals.
Perhaps more amplification is self defeating,
since this is not a PMT. For feedback, I chose
an 0.82 pF capacitor rather than 10 GOhm resistor
as these are very hard to find and expensive. I
think this circuit should work, but would be very
grateful for suggestions for improvements. Please
consider this is a light meter so it should have
a linear response to photon flux.
__________________________________________________________

| |
| high-sensitivity (10mV/fA) current-sensing amplifier |
| by Winfield Hill, 21 Feb 2004 |
| |
| __A1 ,--10.0M---, |
| o means gnd--|+ \ 100nF | __A2 | |
| connect | >--+---||--33k--+--|- \ | |
| in air ,-10k--o-|-_/ | | >---+--- |
| | | G=~20 gnd --|+_/ |
| - - - (o)---||--+-100k-' |
| 2pF | (75k + 50k pot |
| mica 5k for calibration) |
| | G = 10^13 V/A |
| gnd A1 = LMC6061 precision gain |
|________________________________________________________|

Dominic, if you want to experiment with capacitance opamp
feedback, try this circuit I posted, and read the next 40
messages in Roy McCammon's "Mosfet Gate leakage" thread.
http://www.google.com/groups?as_umsgid=c15k53017vl%40drn.newsguy.com

This is an integrator followed by a differentiator, and it
acts like an opamp with a 10,000 Gig-ohm feedback resistor.
__________________________________________________________
| |
| modest-sensitivity (100mV/pA) current amplifier |
| by Winfield Hill, 2 Mar 2004 |
| |
| __A1 ,--10.0M---, |
| o means gnd--|+ \ 0.02uF | __A2 | |
| connect | >--+---||--33k--+--|- \ | |
| in air ,-10k--o-|-_/ | | >---+--- |
| | | gnd --|+_/ |
| - - - (o)---||---------' |
| 2 pF mica |
| G = 10^11 V/A |
| A1 = LMC6061 approximately |
|________________________________________________________|

It'll surely be too sensitive, but we can lower its gain to
act like a 100G resistor. Read the thread to learn a cute
way to reset the 2pF capacitor without changing its value.

Thanks,
- Win

whill_at_picovolt-dot-com

 

[John Larkin]

On 2 Mar 2004 05:53:52 -0800, Winfield Hill
<Winfield_member@newsguy.com> wrote:

Dominic-Luc Webb wrote...

Latest adventure is a minimum component PIN
photometer. Sensitivity is an issue (measuring
from faint stars on telescope). I am thinking
to try two opamps to increase weak signals.
Perhaps more amplification is self defeating,
since this is not a PMT. For feedback, I chose
an 0.82 pF capacitor rather than 10 GOhm resistor
as these are very hard to find and expensive. I
think this circuit should work, but would be very
grateful for suggestions for improvements. Please
consider this is a light meter so it should have
a linear response to photon flux.
__________________________________________________________
| |
| high-sensitivity (10mV/fA) current-sensing amplifier |
| by Winfield Hill, 21 Feb 2004 |
| |
| __A1 ,--10.0M---, |
| o means gnd--|+ \ 100nF | __A2 | |
| connect | >--+---||--33k--+--|- \ | |
| in air ,-10k--o-|-_/ | | >---+--- |
| | | G=~20 gnd --|+_/ |
| - - - (o)---||--+-100k-' |
| 2pF | (75k + 50k pot |
| mica 5k for calibration) |
| | G = 10^13 V/A |
| gnd A1 = LMC6061 precision gain |
|________________________________________________________|

Dominic, if you want to experiment with capacitance opamp
feedback, try this circuit I posted, and read the next 40
messages in Roy McCammon's "Mosfet Gate leakage" thread.
http://www.google.com/groups?as_umsgid=c15k53017vl%40drn.newsguy.com

This is an integrator followed by a differentiator, and it
acts like an opamp with a 10,000 Gig-ohm feedback resistor.
__________________________________________________________
| |
| modest-sensitivity (100mV/pA) current amplifier |
| by Winfield Hill, 2 Mar 2004 |
| |
| __A1 ,--10.0M---, |
| o means gnd--|+ \ 0.02uF | __A2 | |
| connect | >--+---||--33k--+--|- \ | |
| in air ,-10k--o-|-_/ | | >---+--- |
| | | gnd --|+_/ |
| - - - (o)---||---------' |
| 2 pF mica |
| G = 10^11 V/A |
| A1 = LMC6061 approximately |
|________________________________________________________|

It'll surely be too sensitive, but we can lower its gain to
act like a 100G resistor. Read the thread to learn a cute
way to reset the 2pF capacitor without changing its value.

Thanks,
- Win

whill_at_picovolt-dot-com

Win,

There's a circuit in the latest RSI that's nice. The first stage
feedback is a capacitor shunted by a high-value resistor, and the
second stage is a DC coupled amp with an R-C lead network to
compensate for the frequency response of the first stage. It's a nice
compromise: less Johnson noise in the front end, but no reset
requirement.

John

 

[Dominic-Luc Webb]

Thanks bunches. I have plenty of homework to do, but I checked
the specs on the LMC6061 (clearly made for my application). May
I ask if the A2 in the schematics are the second half of a
LMC6062 or a second LMC6061 or some other opamp altogether?

Dominic