AD844AN Analog Devices Inc, AD844AN Datasheet - Page 14

• Manufacturer Part Number: AD844AN
• Description: IC OPAMP CF 60MHZ 80MA 8DIP
• Manufacturer: Analog Devices Inc

Specifications of AD844AN

• Slew Rate: 2000 V/µs
• Rohs Status: RoHS non-compliant
• Amplifier Type: Current Feedback
• Number Of Circuits: 1
• -3db Bandwidth: 60MHz
• Current - Input Bias: 200pA
• Voltage - Input Offset: 50µV
• Current - Supply: 6.5mA
• Current - Output / Channel: 80mA
• Voltage - Supply, Single/dual (±): ±4.5 V ~ 18 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Through Hole
• Package / Case: 8-DIP (0.300", 7.62mm)
• No. Of Amplifiers: 1
• Bandwidth: 60MHz
• No. Of Pins: 8
• Operating Temperature Range: -40°C To +85°C
• Settling Time: 100ns
• Operating Temperature Max: 85°C
• Output Type: -
• Gain Bandwidth Product: -
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

AD844
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
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.
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.
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
.
Rev. F | Page 14 of 20
NONINVERTING GAIN OF 100
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
further extend the bandwidth. Figure 33 shows the small signal
response with C
either ±5 V or ±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
additional offset term due to the bias current at the inverting
input (I
optionally be nulled by the trimming potentiometer shown in
Figure 32.
Figure 32. Noninverting Amplifier Gain = 100, Optional Offset Trim Is Shown
Figure 33. AC Response for Gain = 100, Configuration Shown in Figure 32
46
40
34
28
22
16
BN
4.99Ω
100k
V
C
R2
3nF
), which flows in the feedback resistor (R1). This can
IN
PK
OFFSET
TRIM
PK
= 3 nF, R
3
2
1
4.7Ω
AD844
20Ω
–V
PK
4
FREQUENCY (Hz)
L
8
S
) can optionally be added to
= 500 Ω, and supply voltages of
1M
0.22µF
+V
7
S
4.7Ω
0.22µF
499Ω
6
R1
V
V
S
S
= ±15V
= ±5V
10M
R
L
20M