AD844_07 AD [Analog Devices], AD844_07 Datasheet - Page 9

• Manufacturer Part Number: AD844_07
• Description: 60 MHz, 2000 V/us Monolithic Op Amp
• Manufacturer: AD [Analog Devices]

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 current available to charge the capacitance (about 4.5 pF)
at TZ node, is always proportional to the input error current, and
the slew rate limitations associated with the large signal response
of op amps do not occur. For this reason, the rise and fall times
are almost independent of signal level. In practice, the input
current will eventually cause the mirrors to saturate. When using
± 15 V supplies, this occurs at about 10 mA (or ± 2200 V/µs).
Since signal currents are rarely this large, classical “slew rate”
limitations are absent.
This inherent advantage would be lost if the voltage follower
used to buffer the output were to have slew rate limitations. The
AD844 has been 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
Since current feedback amplifiers are asymmetrical with regard
to their two inputs, performance will differ markedly in nonin-
verting 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 4) is not designed to provide high input voltage
slew rates.
However, good results can be obtained with some care. The
noninverting input will 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).
TPC 20 shows a noninverting amplifier with a gain of 10 and a
bandwidth of 30 MHz. The transient response is shown in
TPCs 23 and 24. To increase the bandwidth at higher gains, a
capacitor can be added across R2 whose value is approximately
the ratio of R1 and R2 times C
Noninverting Gain of 100
The AD844 provides very clean pulse response at high nonin-
verting gains. Figure 5 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 peak-
ing capacitor (C
the bandwidth. Figure 6 shows the small signal response with
C
± 15 V. Gain bandwidth products of up to 900 MHz can be achieved
in this way.
The offset voltage of the AD844 is laser trimmed to the 50 µV
level and exhibits very low drift. In practice, there is an addi-
tional offset term due to the bias current at the inverting input
(I
ally be nulled by the trimming potentiometer shown in Figure 5.
PK
BN
) which flows in the feedback resistor (R1). This can option-
= 3 nF, R
L
PK
= 500 Ω, and supply voltages of either ± 5 V or
) can optionally be added to further extend
t
.
USING THE AD844
Board Layout
As with all high frequency circuits considerable care must be
used in the layout of the components surrounding the AD844.
A ground plane, to which the power supply decoupling capaci-
tors are connected by the shortest possible leads, is essential
to achieving clean pulse response. Even a continuous ground
plane will exhibit finite voltage drops between points on the
plane, and this must be kept in mind in selecting the grounding
points. Generally speaking, decoupling capacitors should be
taken to a point close to the load (or output connector) since
the load currents flow in these capacitors at high frequencies.
The +IN and –IN circuits (for example, a termination resistor
and Pin 3) must be taken to a common point on the ground
plane close to the amplifier package.
Use low impedance capacitors (AVX SR305C224KAA or
equivalent) of 0.22 µF wherever ac coupling is required. Include
either ferrite beads and/or a small series resistance (approxi-
mately 4.7 Ω) in each supply line.
46
40
34
28
22
16
C
100k
4.99
V
PK
R2
IN
3nF
OFFSET
TRIM
+
–
4.7
AD844
20
–V
FREQUENCY – Hz
8
S
1M
0.22 F
+V
S
4.7
499
0.22 F
R1
V
S
=
V
S
15V
=
5V
AD844
10M
R
L
20M