AD795JRZ datasheet(17/20 Pages) AD

17 / 20 page

AD795

Rev. C | Page 17 of 20

USING A T NETWORK

A T network, shown in Figure 49, 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.

10pF

VOUT

100MΩ

1.1kΩ

10kΩ

PHOTODIODE

AD795

VOUT = IDRF (1 +

)

Figure 49. Photodiode Preamp Employing a T Network for Added Gain

A H PROBE BUFFER AMPLIFIER

A typical pH probe requires a buffer amplifier, shown in Figure 50,

to isolate its 106 Ω to 109 Ω source resistance from external

circuitry. 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 50 provides an output voltage

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

by resistor RT, which is a special temperature compensation

resistor, Part Number Q81, 1 kΩ, 1%, 3500 ppm/°C, available

from Tel Labs, Inc.

AD795

OUTPUT

1V/pH UNIT

19.6kΩ

1kΩ

3500ppm/°C

VOS ADJUST

100kΩ

GUARD

–VS

+VS

0.1µF

+VS

–VS

+15V

COM

–15V

PROBE

Figure 50. pH Probe Amplifier

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