AD846
Where:
(RF = 1 kΩ, RS = 10 Ω) it will be 4 MHz. At gains of 3 or
greater, a small capacitor (2 pF–5 pF) connected across the
feedback resistor will help reduce overshoot; but when operating
at noninverting gains below 3, this same capacitance will cause
instability.
RP is the external resistance placed in series with the non-
inverting input
RF is the feedback resistor
RS is the source resistor
INN is the noise current in the inverting input
INP is the noise current in the noninverting input
VN is the input noise voltage.
Typical values for these parameters (@ 1 kHz) in pA/√Hz are:
I
NN = 20, IPN = 6, VN = 2.
Or, referring to the signal input, the equivalent mean-square in-
put voltage noise is:
2 +4kT R
RS
R
2
2 = R I
2 + 1+
VN2 + R I
V
IN
(
)
(
)
F
NN
P
NP
P
F
RS
+ 4kT RS 1+
R
F
Figure 43. AD846 Noninverting Amplifier Configuration
Resistor RP is required for both inverting and noninverting
(follower) operation, to insure stable operation. The amplifier’s
noninverting input current (flowing through RP of 100 Ω) will
typically add less than 300 µV to the AD846’s input offset volt-
age. This can be trimmed-out using the optional network shown
in Figure 44. The following table gives recommended values
for RP.
USING THE COMPENSATION PIN OF THE AD846
Additional compensation may be provided for the AD846 by
applying an external capacitance between Pin 5 and analog
ground (Figure 44). The nominal value of the AD846’s internal
compensation capacitor is 7 pF. For a given value of feedback
resistance (RF), any added external capacitance reduces the
amplifier’s slew rate and bandwidth proportionally.
Recommended
Supply Voltage
Gain (RF/RS)
Value for RF
6 V to 15 V
6 V to 15 V
6 V to 15 V
5 V
1–10
100 Ω
47 Ω
0 Ω
47 Ω
0 Ω
10–20
20–200
1–10
5 V
10–200
Figure 44. AD846 Inverting Amplifier Showing External
Compensation Connection, RP and Optional VOS Trim
In addition to providing for external compensation, Pin 5 may
be used to clamp the output of the amplifier, as shown in
Figure 45. The output can be clamped anywhere within the
output range (approximately 10 V) of the amplifier. The in-
put should also be clamped as a precaution against damaging the
amplifier’s input transistors.
Figure 42. Op Amp Simplified Noise Model
NONINVERTING GAIN OPERATION
The AD846 can be used as a noninverting amplifier or voltage
follower, operating at gains between 1 and 200. A minimum
value of RF equal to 1 kΩ should be employed. For low gains
(1 to 2), the input signal should be applied to the AD846’s non-
inverting input through a 100 Ω series resistor; this will help
reduce peaking. The best transient response will occur when the
amplifier’s output level is below 5 V peak to peak.
At closed-loop gains of 3 or more, the input resistor is not re-
quired unless peak signals greater than 3 V will be applied. The
amplifier’s bandwidth can be determined by using the inverting
amplifier’s bandwidth equation or from Figure 41. For example,
at a gain of + 10 (RF = 1 kΩ, RS = 100 Ω) the bandwidth of the
AD846 will be approximately 33 MHz; at a gain of +100,
Figure 45. AD846 Used as a Clamped Amplifier
REV. C
–10–
ADI [ ADI ]
