3

Typical Applications

Picoammeter Circuit

The exceptionally low input current (typically 0.2pA) makes

the CA3420 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

CA3420 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 CA3420

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 coefficient error term.

Supply current in the standby position with the meter

undeflected is 300

µA. At full-scale deflection this current

rises to 800

µA. Carbon-zinc battery life should be in excess

of 1,000 hours.

Electrical Specifications

PARAMETER

SYMBOL

TEST

CONDITIONS

CA3420

CA3420A

UNITS

MIN

TYP

MAX

MIN

TYP

MAX

Input Offset Voltage

-

5

10

-

2

5

mV

Input Offset Current (Note 4)

-

0.03

4

-

0.03

4

pA

Input Current (Note 4)

-

0.05

5

-

0.05

5

pA

Large Signal Voltage Gain

10

100

-

20

100

-

kV/V

80

100

-

86

100

-

dB

Common Mode Rejection Ratio

CMRR

-

100

320

-

100

320

µV/V

70

80

-

70

80

-

dB

Common Mode Input Voltage Range

8.5

9.3

-

9.0

9.3

-

V

-10

-10.3

-

-10

-10.3

-

V

Power Supply Rejection Ratio

PSRR

-

32

320

-

32

320

µV/V

70

90

-

70

90

-

dB

Max Output Voltage

9.7

9.9

-

9.7

9.9

-

V

-9.7

-9.85

-

-9.7

-9.85

-

V

Supply Current

I+

-

450

1000

-

450

1000

µA

Device Dissipation

-

9

14

-

9

14

mW

Input Offset Voltage Temperature Drift

-

4-

-

4-

NOTE:

4. The maximum limit represents the levels obtainable on high speed automatic test equipment. Typical values are obtained under laboratory conditions.

3

CA3420

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

µA

±50pA

±15pA

±5pA

±1.5pA

11k

Ω

1.5k

Ω, 1%

1.5k

Ω

1k

Ω

430

Ω, 1%

150

Ω, 1%

68

Ω

1%

-

+

FIGURE 1. PICOAMMETER CIRCUIT

2

CA3420

4

500-0-500

7

+1.5V

5

1

3

-1.5V

10M

Ω

22M

Ω

BATTERY

RETURNS

10k

Ω

6

M

µA

±500mV

±150mV

±50mV

±15mV

1.1k

Ω

1.5k

Ω, 1%

1.5k

Ω

1k

Ω

430

Ω, 1%

150

Ω, 1%

68

Ω

1%

-

+

100pF

FIGURE 2. HIGH INPUT RESISTANCE VOLTMETER

CA3420, CA3420A