6

FN1925.6

December 8, 2009

Typical Applications

Picoammeter Circuit

The exceptionally low input current (typically 0.2pA) makes

the CA5420A highly suited for use in a picoammeter circuit.

With only a single 10G

Ω resistor, this circuit covers the range

from

±1.5pA. Higher current ranges are possible with suitable

switching techniques and current scaling resistors. Input

transient protection is provided by the 1M

Ω resistor in series

with the input. Higher current ranges require that this resistor

be reduced. The 10M

Ω resistor connected to pin 2 of the

CA5420A decouples the potentially high input capacitance

often associated with lower current circuits and reduces the

tendency for the circuit to oscillate under these conditions.

High Input Resistance Voltmeter

Advantage is taken of the high input impedance of the

CA5420A in a high input resistance DC voltmeter. Only two

1.5V “AA” type penlite batteries power this exceedingly

high-input resistance (>1,000,000M

Ω) DC voltmeter. Full-scale

deflection is

±500mV, ±150mV, and ±15mV. Higher voltage

ranges are easily added with external input voltage attenuator

networks.

The meter is placed in series with the gain network, thus

eliminating the meter temperature co-efficient error term.

Supply current in the standby position with the meter

undeflected is 300µA. At full-scale deflection this current

rises to 800µA. Carbon-zinc battery life should be in excess

of 1,000 hours.

3

CA5420A

4

500-0-500

7

+1.5V

5

1

2

-1.5V

10pF

10G

Ω

10M

Ω

1M

Ω

BATTERY

RETURNS

10k

Ω

6

M

µ

A

±50pA

±15pA

±5pA

±1.5pA

11k

Ω

1.5k

Ω

1.5k

Ω

1k

Ω

430

Ω

150

Ω

68

Ω

1%

-

+

1%

1%

1%

FIGURE 1. PICOAMMETER CIRCUIT

2

CA5420A

4

500-0-500

7

+1.5V

5

1

3

-1.5V

10M

Ω

22M

Ω

BATTERY

RETURNS

10k

Ω

6

M

µ

A

±500mV

±150mV

±50mV

±15mV

1.1k

Ω

1.5k

Ω

1.5k

Ω

1k

Ω

430

Ω

150

Ω

68

Ω

1%

-

+

100pF

1%

1%

1%

FIGURE 2. HIGH INPUT RESISTANCE VOLTMETER

Typical Performance Curves

FIGURE 3. OUTPUT VOLTAGE SWING AND COMMON MODE

INPUT VOLTAGE RANGE vs SUPPLY VOLTAGE

FIGURE 4. OUTPUT VOLTAGE vs LOAD SOURCING CURRENT

10

SUPPLY VOLTAGE (V)

15

-1.0

5

1

0

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1.0

V

+ = 5V

10

1

0.1

0.001

LOAD (SOURCING) CURRENT (mA)

V- = 0V

1000

100

10

V

+ = 20V

V

+ = 10V

V

+ = 2V

CA5420A