AD546

REV. A

–12–

Figure 42. Photodiode Preamp Noise Sources’ Spectral

Density vs. Frequency

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

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

6

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

9

Ω. Input offset voltage (which can

be trimmed) will be below 2 mV. Refer to AD549 data sheet for

temperature compensated pH probe amplifier circuit.

Figure 43. Photodiode Array Processor

Figure 44. pH Probe Amplifier

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).

Mini-DIP (N) Package