
CA5420A
6
FN1925.8
September 25, 2013
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 300A. At full-scale deflection this current rises to
800A. 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
A
±50pA
±15pA
±5pA
±1.5pA
11k
Ω
1.5k
Ω
1.5k
Ω
1k
Ω
430
Ω
150
Ω
68
Ω
1%
-
+
1%
FIGURE 2. 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
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%
FIGURE 3. HIGH INPUT RESISTANCE VOLTMETER
Typical Performance Curves
FIGURE 4. OUTPUT VOLTAGE SWING AND COMMON MODE
INPUT VOLTAGE RANGE vs SUPPLY VOLTAGE
FIGURE 5. OUTPUT VOLTAGE vs LOAD SOURCING CURRENT
RL = 100kΩ
10
SUPPLY VOLTAGE (V)
15
TA = +25°C
-1.0
5
1
0
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1.0
VO+
VICR-
VICR+
INPUT
AND
OUTPUT
V
O
LT
AGE
E
X
CURSIONS
FROM
THE
P
O
SI
TI
VE
AND
NEG
A
TI
VE
SUPP
LY
VO
LT
AGE
(V)
VO-
V+ = 5V
10
1
0.1
0.001
LOAD (SOURCING) CURRENT (mA)
V- = 0V
TA = +25°C
1000
100
10
OUTP
UT
S
TAGE
TR
ANS
IST
O
R
SA
TUR
A
T
ION
VOL
TA
G
E,
Q19
(m
V)
V+ = 20V
V+ = 10V
V+ = 2V