AD841
Data Sheet
Rev. C | Page 10 of 16
THEORY OF OPERATION
OFFSET NULLING
The input offset voltage of the
AD841 is very low for a high
speed op amp, but if additional nulling is required, the circuit
Figure 28. Offset Nulling (PDIP Pinout)
INPUT CONSIDERATIONS
An input resistor (RIN in Figure 25) is recommended in circuits where the input to t
he AD841 is subjected to transient or
continuous overload voltages exceeding the ±6 V maximum
differential limit. This resistor provides protection for the input
transistors by limiting the maximum current that can be forced
into the input.
For high performance circuits it is recommended that a resistor
errors by matching the impedance at each input. The output
voltage error caused by the offset current is more than an order
of magnitude less than the error present if the bias current error
is not removed.
Settling time is defined as the interval of time from the
application of an ideal step function input until the closed-loop
amplifier output has entered and remains within a specified
error band.
This definition encompasses the major components, which
comprise settling time. They include
Propagation delay through the amplifier
Slewing time to approach the final output value
The time of recovery from the overload associated
with slewing
Linear settling to within the specified error band
Expressed in these terms, the measurement of settling time
is obviously a challenge and needs to be done accurately to
assure the user that the amplifier is worth consideration for
the application.
Figure 30. Settling Time Test Circuit
Measurement of the 0.01% settling in 110 ns was accomplished
by amplifying the error signal from a false summing junction
with a very high speed proprietary hybrid error amplifier
specially designed to enable testing of small settling errors.
The device under test was driving a 500 Ω load. The input to
the error amp is clamped to avoid possible problems associated
with the overdrive recovery of the oscilloscope input amplifier.
The error amp gains the error from the false summing junction
by 10, and it contains a gain vernier to fine trim the gain.
11
12
3
4
5
10
–
+
AD841
+VS
6
–VS
2.2F
0.1F
RL
INPUT
OUTPUT
100
1
1340-
028
100
90
10
0%
10mV
5V
20ns
OUTPUT ERROR:
0.02%/DIV
OUTPUT:
5V/DIV
1
1340-
029
DDD5109
FLAT-TOP
PULSE
GENERATOR
11
6
4
5
10
–
+
AD841
+15V
–15V
2.2F
0.1F
2.2F
0.1F
499
50
499
1k
ERROR
AMP
(×10)
TEK
7A13
T
EK
7603
OS
C
ILLOS
C
OP
E
TEK
7A18
HP6263
FET PROBE
TEK P6201
1
1340-
030