AD795

REV. A

–14–

FREQUENCY – Hz

100

1k

10k

100k

10

1

10nV

100nV

1

µV

10

µV

SIGNAL BANDWIDTH

NO FILTER

WITH FILTER

en

&

i

f

in

en

Figure 46. Voltage Noise Spectral Density of the Circuit of

Figure 45 With and Without an Output Filter

PHOTODIODE

AD795

10pF

V

OUT

10

Ω

8

10

k

Ω

Rf

V

= I Rf (1+

)

OUT

D

R

1.1k

Ω

i

Figure 47. A Photodiode Preamp Employing a “T”

Network for Added Gain

Using a “T” Network

A “T” network, shown in Figure 47, can be used to boost the

effective transimpedance of an I-to-V converter, for a given

feedback resistor value. However, amplifier noise and offset

voltage contributions are also amplified by the “T” network

gain. A low noise, low offset voltage amplifier, such as the

AD795, is needed for this type of application.

A pH Probe Buffer Amplifier

A typical pH probe requires a buffer amplifier to isolate its 10

6

to 10

9

Ω source resistance from external circuitry. Just such an

amplifier is shown in Figure 48. The low input current of the

AD795 allows the voltage error produced by the bias current

and electrode resistance to be minimal. The use of guarding,

shielding, high insulation resistance standoffs, and other such

standard methods used to minimize leakage are all needed to

maintain the accuracy of this circuit.

The slope of the pH probe transfer function, 50 mV per pH

unit at room temperature, has a +3300 ppm/

°C temperature

coefficient. The buffer of Figure 48 provides an output voltage

equal to 1 volt/pH unit. Temperature compensation is provided

by resistor RT which is a special temperature compensation

resistor, part number Q81, 1 k

Ω, 1%, +3500 ppm/°C, available

from Tel Labs Inc.

GUARD

AD795

V

ADJUST

100k

Ω

OS

PH

PROBE

+V

S

OUTPUT

1VOLT/pH UNIT

19.6k

Ω

RT

1k

Ω

+3500ppm/

°C

0.1

µF

0.1

µF

+15V

COM

–15V

+V

S

1

3

2

4

5

6

7

8

Figure 48. A pH Probe Amplifier