AD546JN AD [Analog Devices], AD546JN Datasheet - Page 11

• Manufacturer Part Number: AD546JN
• Description: 1 pA Monolithic Electrometer Operational Amplifier
• Manufacturer: AD [Analog Devices]

tacting the device under test. Rigid Teflon coaxial cable is used
to make connections to all high impedance nodes. The use of
rigid coax affords immunity to error induced by mechanical vi-
bration and provides an outer conductor for shielding. The en-
tire circuit is enclosed in a grounded metal box.
The test apparatus is calibrated without a device under test
present. A five minute stabilization period after the power is
turned on is required. First, V
These voltages are the errors caused by offset voltages and leak-
age currents of the current to voltage converters.
Once measured, these errors are subtracted from the readings
taken with a device under test present. Amplifier B closes the
feedback loop to the device under test, in addition to providing
current to voltage conversion. The offset error of the device un-
der test appears as a common-mode signal and does not affect
the test measurement. As a result, only the leakage current of
the device under test is measured.
Although a series of devices can be tested after only one calibra-
tion measurement, calibration should be updated periodically to
compensate for any thermal drift of the current-to-voltage con-
verters or changes in the ambient environment. Laboratory re-
sults have shown that repeatable measurements within 10 fA can
be realized when this apparatus is properly implemented. These
results are achieved in part by the design of the circuit, which
eliminates relays and other parasitic leakage paths in the high
impedance signal lines, and in part by the inherent cancellation
of errors through the calibration and measurement procedure.
PHOTODIODE INTERFACE
The AD546’s 1 pA current and low input offset voltage make it
a good choice for very sensitive photodiode preamps (Figure
39). The photodiode develops a signal current, I
where P is light power incident on the diode’s surface in watts
and R is the photodiode responsivity in amps/watt. R
the signal current to an output voltage:
DC error sources and an equivalent circuit for a small area
(0.2 mm square) photodiode are indicated in Figure 40.
REV. A
Figure 39. Photodiode Preamp
V
V
V
V
ERR1
ERR2
A
X
– V
– V
= 10 (V
= 10 (V
ERR1
ERR2
V
OUT
I
S
= 10[RSa
= 10[RSb
= R
ERR1
OS
OS
= R
A – I
B – I
F
and V
P
B
B
I
A
B
S
ERR2
I
I
B
B
RSa)
RSb)
(+)]
(–)]
are measured.
S
, equal to:
F
converts
–11–
Input current, I
proportional to the feedback resistance:
The op amp’s input voltage offset will cause an error current
through the photodiode’s shunt resistance, R
The error current will result in an error voltage (V
amplifier’s output equal to:
Given typical values of photodiode shunt resistance (on the or-
der of 10
feedback resistance is used. Also, R
perature, as photodiode shunt resistance typically drops by a
factor of two for every 10 C rise in temperature. An op amp
with low offset voltage and low drift helps maintain accuracy.
Photodiode Preamp Noise
Noise limits the signal resolution obtainable with the preamp.
The output voltage noise divided by the feedback resistance is
the minimum current signal that can be detected. This mini-
mum detectable current divided by the responsivity of the pho-
todiode represents the lowest light power that can be detected
by the preamp.
Noise sources associated with the photodiode, amplifier, and
feedback resistance are shown in Figure 41; Figure 42 is the
voltage spectral density versus frequency plot of each of the
noise source’s contribution to the output voltage noise (circuit
parameters in Figure 40 are assumed). Each noise source’s rms
contribution to the total output voltage noise is obtained by in-
tegrating the square of its spectral density function over fre-
quency. The rms value of the output voltage noise is the square
root of the sum of all contributions. Minimizing the total area
under these curves will optimize the preamplifier’s resolution for
a given bandwidth.
Figure 40. Photodiode Preamp DC Error Sources
Figure 41. Photodiode Preamp Noise Sources
9
), R
B
F
, will contribute an output voltage error, V
/R
S
can be greater than one, especially if a large
V
E2
V
= (1 +R
I = V
E1
= I
OS
B
F
/R
/R
F
R
/R
S
S
) V
F
S
will increase with tem-
OS
S
:
AD546
E2
) at the
E1
,