AD546JN AD [Analog Devices], AD546JN Datasheet - Page 9

• Manufacturer Part Number: AD546JN
• Description: 1 pA Monolithic Electrometer Operational Amplifier
• Manufacturer: AD [Analog Devices]

input offset voltage drift with temperature is not affected. How-
ever, variation of the power supply voltages will cause offset
shifts.
AC RESPONSE WITH HIGH VALUE SOURCE AND
FEEDBACK RESISTANCE
Source and feedback resistances greater than 100 k will
magnify the effect of input capacitances (stray and inherent to
the AD546) on the ac behavior of the circuit. The effects of
common-mode and differential-input capacitances should be
taken into account since the circuit’s bandwidth and stability
can be adversely affected.
In a follower, the source resistance, R
mode capacitance, C
capacitance inherent to the AD546), form a pole that limits cir-
cuit bandwidth to 1/2 R
pulse response from a 1 M source resistance with the
amplifier’s input pin isolated from the board, only the effect of
the AD546’s input common-mode capacitance is seen.
In an inverting configuration, the differential input capacitance
forms a pole in the circuit’s loop transmission. This can create
peaking in the ac response and possible instability. A feedback
capacitance can be used to stabilize the circuit. The inverter
pulse response with R
isolated from the board appears in Figure 32. Figure 33 shows
the response of the same circuit with a 1 pF feedback capaci-
tance. Typical differential input capacitance for the AD546
is 1 pF.
REV. A
Figure 31. Follower Pulse Response from 1 M Source
Resistance
Figure 30. Alternate Offset Null Circuit for Inverter
S
F
(including capacitance due to board and
and R
S
C
S
. Figure 31 shows the follower
S
equal to 1 M , and the input pin
S
, and input common-
–9–
Figure 32. Inverter Pulse Response with 1 M Source and
Feedback Resistance
Figure 33. Inverter Pulse Response with 1 M Source and
Feedback Resistance, 1 pF Feedback Capacitance
COMMON-MODE INPUT VOLTAGE OVERLOAD
The rated common-mode input voltage range of the AD546 is
from 3 V less than the positive supply voltage to 5 V greater
than the negative supply voltage. Exceeding this range will de-
grade the amplifier’s CMRR. Driving the common-mode volt-
age above the positive supply will cause the amplifier’s output to
saturate at the upper limit of output voltage. Recovery time is
typically 2 s after the input has been returned to within the
normal operating range. Driving the input common mode volt-
age within 1 V of the negative supply causes phase reversal of
the output signal. In this case, normal operation is typically
resumed within 0.5 ms of the input voltage returning within
range.
DIFFERENTIAL INPUT VOLTAGE OVERLOAD
A plot of the AD546’s input current versus differential input
voltage (defined as V
Figure 34. Input Current vs. Differential Input Voltage
IN
+ –V
IN
–) appears in Figure 34. The
AD546