Datasheet

AD844
Rev. F | Page 14 of 20
NONINVERTING GAIN OF 100
It is important to understand that the low input impedance at
the inverting input is locally generated and does not depend on
feedback. This is very different from the virtual ground of a
conventional operational amplifier used in the current summing
mode, which is essentially an open circuit until the loop settles.
In the AD844, transient current at the input does not cause
voltage spikes at the summing node while the amplifier is
settling. Furthermore, all of the transient current is delivered
to the slewing (TZ) node (Pin 5) via a short signal path (the
grounded base stages and the wideband current mirrors).
The AD844 provides very clean pulse response at high
noninverting gains. Figure 32 shows a typical configuration
providing a gain of 100 with high input resistance. The feedback
resistor is kept as low as practicable to maximize bandwidth,
and a peaking capacitor (C
PK
) can optionally be added to
further extend the bandwidth. Figure 33 shows the small signal
response with C
PK
= 3 nF, R
L
= 500 Ω, and supply voltages of
either ±5 V or ±15 V. Gain bandwidth products of up to
900 MHz can be achieved in this way.
The current available to charge the capacitance (about 4.5 pF) at
the TZ node is always proportional to the input error current,
and the slew rate limitations associated with the large signal
response of the op amps do not occur. For this reason, the rise
and fall times are almost independent of signal level. In practice,
the input current eventually causes the mirrors to saturate.
When using ±15 V supplies, this occurs at about 10 mA (or
±2200 V/μs). Because signal currents are rarely this large,
classical slew rate limitations are absent.
The offset voltage of the AD844 is laser trimmed to the 50 μV
level and exhibits very low drift. In practice, there is an
additional offset term due to the bias current at the inverting
input (I
BN
), which flows in the feedback resistor (R1). This can
optionally be nulled by the trimming potentiometer shown in
Figure 32.
OFFSET
TRIM
C
PK
3nF
20Ω
4.7Ω
0.22µF
0.22µF
R
L
V
IN
+
V
S
–V
S
AD844
R1
499Ω
R2
4.99Ω
4.7Ω
1
2
3
8
7
4
6
00897-032
This inherent advantage is lost if the voltage follower used
to buffer the output has slew rate limitations. The AD844 is
designed to avoid this problem, and as a result, the output
buffer exhibits a clean large signal transient response, free
from anomalous effects arising from internal saturation.
RESPONSE AS A NONINVERTING AMPLIFIER
Because current feedback amplifiers are asymmetrical with
regard to their two inputs, performance differs markedly in
noninverting and inverting modes. In noninverting modes, the
large signal high speed behavior of the AD844 deteriorates at
low gains because the biasing circuitry for the input system (not
shown in Figure 31) is not designed to provide high input
voltage slew rates.
Figure 32. Noninverting Amplifier Gain = 100, Optional Offset Trim Is Shown
FREQUENCY (Hz)
GAIN (dB)
46
16
100k 1M 20M10M
40
34
28
22
00897-040
V
S
= ±5V
V
S
= ±15V
However, good results can be obtained with some care. The
noninverting input does not tolerate a large transient input; it
must be kept below ±1 V for best results. Consequently, this
mode is better suited to high gain applications (greater than
×10). Figure 23 shows a noninverting amplifier with a gain of 10
and a bandwidth of 30 MHz. The transient response is shown in
Figure 26 and Figure 27. To increase the bandwidth at higher
gains, a capacitor can be added across R2 whose value is
approximately (R1/R2) × C
t
.
Figure 33. AC Response for Gain = 100, Configuration Shown in Figure 32