AD549

Rev. H | Page 16 of 20

Frequency compensation is provided by R11, R12, C1, and C2.

The bandwidth of the circuit is 300 kHz at input signals greater

than 50 μA; bandwidth decreases smoothly with decreasing

signal levels.

To trim the circuit, set the input currents to 10 μA and trim the

A3 offset using the trim potentiometer of the amplifier for the

equal 1 V by trimming R10. Additional offset trims on Ampli-

fier A1 and Amplifier A2 can be used to increase the voltage

input accuracy and dynamic range.

The very low input current of the AD549 makes this circuit

useful over a very wide range of signal currents. The total input

current (which determines the low level accuracy of the circuit)

is the sum of the amplifier input current, the leakage across the

compensating capacitor (negligible if a polystyrene or Teflon

capacitor is used), and the collector-to-collector and collector-

to-base leakages of one side of the dual log transistors. The

magnitudes of these last two leakages depend on the amplifier

input offset voltage and are typically less than 10 fA with 1 mV

offsets. The low level accuracy is limited primarily by the

amplifier input current, only 60 fA maximum when the

AD549L is used.

The effects of the emitter resistance of Q1 and Q2 can degrade

circuit accuracy at input currents above 100 μA. The networks

composed of R13, D1, R16, R14, D2, and R17 compensate for

these errors, so that this circuit has less than a 1% log confor-

mance error at 1 mA input currents. The correct value for R13

and R14 depends on the type of log transistors used. The 49.9 kΩ

resistors were chosen for use with LM394 transistors. Smaller

resistance values are needed for smaller log transistors.

TEMPERATURE COMPENSATED pH PROBE

AMPLIFIER

A pH probe can be modeled as an mV-level voltage source

with a series source resistance dependent on the electrode

composition and configuration. The glass bulb resistance of a

therefore important to select an amplifier with low enough

input currents such that the voltage drop produced by the

amplifier input bias current and the electrode resistance does

not become an appreciable percentage of a pH unit.

The circuit in Figure 46 illustrates the use of the AD549 as a pH

probe amplifier. As with other electrometer applications, the use of

guarding, shielding, and Teflon standoffs is necessary to capitalize

on the AD549 low input current. If an AD549L (60 fA maximum

input current) is used, the error contributed by the input current is

Input offset voltages (which can be trimmed) are below 0.5 mV.

3

2

6

5

1

4

A3

AD549

R10

2kΩ

10kΩ

OUTPUT

OFFSET

SCALE

FACTOR

ADJ

R9

14.3kΩ

R8

1kΩ

*

*

R4

20kΩ

4.99kΩ

R6

20kΩ

R7

15kΩ

R15

1kΩ

R5

20kΩ

R3

20kΩ

0.1µF

0.1µF

FOR EACH AMPLIFIER

PIN 7

PIN 4

2

3

6

5

1

4

A2

AD549

OFFSET

10kΩ

D4

C2

100pF

R2

10kΩ

Q2

B

D2

D1

2

6

5

1

4

A1

AD549

OFFSET

10kΩ

D3

3

R1

10kΩ

Q1

A

C1

100pF

R11

4.99kΩ

R14

49.9kΩ

R16

10Ω

R17

10Ω

R13

49.9kΩ

Q1, Q2 = LM394

DUAL LOG TRANSISTORS

D1, D4 1N4148 DIODES

R8, R15 1kΩ + 350 ppm/°C TC RESISTOR

ALL OTHER RESISTORS ARE 1% METAL FILM

Figure 45. Log Ratio Amplifier