Datasheet

3
FN1320.9
October 4, 2005
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 10GΩ 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 1MΩ resistor in series
with the input. Higher current ranges require that this resistor
be reduced. The 10MΩ 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 (>1,000,000MΩ) 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 300µA. At full-scale deflection this current
rises to 800µA. Carbon-zinc battery life should be in excess
of 1,000 hours.
Electrical Specifications For Equipment Design, at V
SUPPLY
= ±10V, T
A
= 25°C, Unless Otherwise Specified
PARAMETER SYMBOL
TEST
CONDITIONS MIN TYP MAX UNITS
Input Offset Voltage |V
IO
| - 5 10 mV
Input Offset Current (Note 4) |I
IO
| - 0.03 4 pA
Input Current (Note 4) |I
I
| - 0.05 5 pA
Large Signal Voltage Gain A
OL
R
L
= 10kΩ 10 100 - kV/V
80 100 - dB
Common Mode Rejection Ratio CMRR - 100 320 µV/V
70 80 - dB
Common Mode Input Voltage Range V
lCR
+8.59.3- V
V
lCR
- -10 -10.3 - V
Power Supply Rejection Ratio PSRR ∆V
IO
/∆V - 32 320 µV/V
70 90 - dB
Max Output Voltage V
OM
+R
L
= ∞ 9.7 9.9 - V
V
OM
- -9.7 -9.85 - V
Supply Current I+ - 450 1000 µA
Device Dissipation P
D
-914 mW
Input Offset Voltage Temperature Drift ∆V
lO
/∆T- 4-µV/°C
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
10GΩ
10MΩ1MΩ
BATTERY
RETURNS
10kΩ
6 M
µA
±50pA
±15pA
±5pA
±1.5pA
11kΩ
1.5kΩ, 1%
1.5kΩ
1kΩ
430Ω, 1%
150Ω, 1%
68Ω
1%
-
+
FIGURE 1. PICOAMMETER CIRCUIT
2
CA3420
4
500-0-500
7
+1.5V
5
1
3
-1.5V
10MΩ22MΩ
BATTERY
RETURNS
10kΩ
6 M
µA
±500mV
±150mV
±50mV
±15mV
1.1kΩ
1.5kΩ, 1%
1.5kΩ
1kΩ
430Ω, 1%
150Ω, 1%
68Ω
1%
-
+
100pF
FIGURE 2. HIGH INPUT RESISTANCE VOLTMETER
CA3420