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AD648S

Dual Precision, Low Power BiFET Op Amp

Analog Devices 

AD648

The AD648 in this configuration provides a 700 kHz small sig-

nal bandwidth and 1.8 V/µs typical slew rate. The 33 pF capaci-

tor across the feedback resistor optimizes the circuit’s response.

The oscilloscope photos in Figures 26a and 26b show small and

large signal outputs of the circuit in Figure 24. Upper traces

show the input signal VIN. Lower traces are the resulting output

voltage with the DAC’s digital input set to all 1s. The circuit

settles to ± 0.01% for a 20 V input step in 14 µs.

Figure 26a. Response to ±20 V p-p Reference Square

Wave

DUAL PHOTODIODE PREAMP

The performance of the dual photodiode preamp shown in Fig-

ure 27 is enhanced by the AD648’s low input current, input

voltage offset, and offset voltage drift. Each photodiode sources

a current proportional to the incident light power on its surface.

RF converts the photodiode current to an output voltage equal

to RF × IS.

An error budget illustrating the importance of low amplifier in-

put current, voltage offset, and offset voltage drift to minimize

output voltage errors can be developed by considering the

equivalent circuit for the small (0.2 mm2 area) photodiode

shown in Figure 27. The input current results in an error pro-

portional to the feedback resistance used. The amplifier’s offset

will produce an error proportional to the preamp’s noise gain

(1+RF/RSH), where RSH is the photodiode shunt resistance. The

amplifier’s input current will double with every 10°C rise in

temperature, and the photodiode’s shunt resistance halves with

every 10°C rise. The error budget in Figure 28 assumes a room

temperature photodiode RSH of 500 MΩ, and the maximum in-

put current and input offset voltage specs of an AD648C.

The capacitance at the amplifier’s negative input (the sum of the

photodiode’s shunt capacitance, the op amp’s differential input

capacitance, stray capacitance due to wiring, etc.) will cause a

rise in the preamp’s noise gain over frequency. This can result in

excess noise over the bandwidth of interest. CF reduces the

noise gain “peaking” at the expense of signal bandwidth.

Figure 26b. Response to ±100 mV p-p Reference Square

Wave

Figure 27. A Dual Photodiode Pre-Amp

TEMP

؇C

–25

0

+25

+50

+75

+85

RSH

VOS

(M⍀) (␮V)

15,970 150

2,830 225

500 300

88.5 375

15.6 450

7.8

480

(1 + RF/RSH) VOS

151 ␮V

233 ␮V

360 ␮V

800 ␮V

3.33 mV

6.63 mV

IB

(pA)

0.30

2.26

10.00

56.6

320

640

IBRF

30 ␮V

262 ␮V

1.0 mV

5.6 mV

32 mV

64 mV

TOTAL

181 ␮V

495 ␮V

1.36 mV

6.40 mV

35.3 mV

70.6 mV

Figure 28. Photodiode Pre-Amp Errors Over Temperature

–8–

REV. C

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