AD546JN AD [Analog Devices], AD546JN Datasheet - Page 12

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

AD546
The photodiode preamp in Figure 39 can detect a signal current
of 26 fA rms at a bandwidth of 16 Hz, which assuming a photo-
diode responsivity of 0.5 A/W, translates to a 52 fW rms mini-
mum detectable power. The photodiode used has a high source
resistance and low junction capacitance. C
width with R
multiplies the op amp’s input voltage noise contribution. A
single pole filter at the amplifier’s output limits the op amp’s
output voltage noise bandwidth to 26 Hz, a frequency compa-
rable to the signal bandwidth. This greatly improves the
preamplifier’s signal to noise ratio (in this case, by a factor of
three).
Photodiode Array Processor
The AD546 is a cost effective preamp for multichannel applica-
tions, such as amplifying signals from photo diode arrays, as il-
lustrated in Figure 43. An AD546 preamp converts each of the
diodes’ output currents to a voltage. An 8 to 1 multiplexer
switches a particular preamp output to the input of an AD1380
16-bit sampling ADC. The output of the ADC can be displayed
or put onto a databus. Additional preamps and muxes can be
added to handle larger arrays. Layout of multichannel circuits is
critical. Refer to “PC board notes” for guidance.
pH PROBE AMPLIFIER
A pH probe can be modeled as a mV-level voltage source with a
series source resistance dependent upon the electrode’s compo-
sition and configuration. The glass bulb resistance of a typical
pH electrode pair falls between 10
important to select an amplifier with low enough input currents
such that the voltage drop produced by the amplifier’s input
bias current and the electrode resistance does not become an
appreciable percentage of a pH unit.
The circuit in Figure 44 illustrates the use of the AD546 as a
pH probe amplifier. As with other electrometer applications, the
use of guarding, shielding, Teflon standoffs, etc., is a must in
order to capitalize on the AD546’s low input current. If an
AD546J (1 pA max input current) is used, the error contributed
by input current will be held below 10 mV for pH electrode
source impedances up to 10
be trimmed) will be below 2 mV. Refer to AD549 data sheet for
temperature compensated pH probe amplifier circuit.
Figure 42. Photodiode Preamp Noise Sources’ Spectral
Density vs. Frequency
F
and also limits the “peak” in the noise gain that
9
. Input offset voltage (which can
6
and 10
F
sets the signal band-
9
. It is, therefore,
–12–
Figure 43. Photodiode Array Processor
Figure 44. pH Probe Amplifier
Dimensions shown in inches and (mm).
OUTLINE DIMENSIONS
Mini-DIP (N) Package
REV. A