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 10G
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
resistor in series
with the input. Higher current ranges require that this resistor
be reduced. The 10M
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
) 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
A. At full-scale deflection this current
rises to 800
A. Carbon-zinc battery life should be in excess
of 1,000 hours.
Electrical Specifications
For Equipment Design, at VSUPPLY = ±10V, TA = 25°C, Unless Otherwise Specified
PARAMETER
SYMBOL
TEST
CONDITIONS
MIN
TYP
MAX
UNITS
Input Offset Voltage
|VIO|
-
5
10
mV
Input Offset Current (Note 4)
|IIO|
-
0.03
4
pA
Input Current (Note 4)
|II|
-
0.05
5
pA
Large Signal Voltage Gain
AOL
RL = 10k
10
100
-
kV/V
80
100
-
dB
Common Mode Rejection Ratio
CMRR
-
100
320
V/V
70
80
-
dB
Common Mode Input Voltage Range
VlCR+8.5
9.3
-
V
VlCR-
-10
-10.3
-
V
Power Supply Rejection Ratio
PSRR
VIO/V
-
32
320
V/V
70
90
-
dB
Max Output Voltage
VOM+RL = ∞
9.7
9.9
-
V
VOM-
-9.7
-9.85
-
V
Supply Current
I+
-
450
1000
A
Device Dissipation
PD
-9
14
mW
Input Offset Voltage Temperature Drift
VlO/T-
4
-
V/°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
10G
10M
1M
BATTERY
RETURNS
10k
6
M
A
±50pA
±15pA
±5pA
±1.5pA
11k
1.5k
, 1%
1.5k
1k
430
, 1%
150
, 1%
68
1%
-
+
FIGURE 1. PICOAMMETER CIRCUIT
2
CA3420
4
500-0-500
7
+1.5V
5
1
3
-1.5V
10M
22M
BATTERY
RETURNS
10k
6
M
A
±500mV
±150mV
±50mV
±15mV
1.1k
1.5k
, 1%
1.5k
1k
430
, 1%
150
, 1%
68
1%
-
+
100pF
FIGURE 2. HIGH INPUT RESISTANCE VOLTMETER
CA3420