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 photodiode responsivity of 0.5 A/W, translates to a 52 fW rms minimum detectable power. The photodiode used has a high source resistance and low junction capacitance. CF sets the signal bandwidth with RF and also limits the “peak” in the noise gain that 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 comparable to the signal bandwidth. This greatly improves the preamplifier’s signal to noise ratio (in this case, by a factor of three).
Figure 43. Photodiode Array Processor
Photodiode Array Processor
The AD546 is a cost effective preamp for multichannel applications, such as amplifying signals from photo diode arrays, as illustrated 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.
Figure 44. pH Probe Amplifier
OUTLINE DIMENSIONS Dimensions shown in inches and (mm).
pH PROBE AMPLIFIER Mini-DIP (N) Package
PRINTED IN U.S.A.
A pH probe can be modeled as a mV-level voltage source with a series source resistance dependent upon the electrode’s composition and configuration. The glass bulb resistance of a typical pH electrode pair falls between 106 Ω and 109 Ω. It is, therefore, 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 109 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.