Datasheet AD844 (Analog Devices) - 14

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AD844 Data Sheet
It is important to understand that the low input impedance at
NONINVERTING GAIN OF 100
the inverting input is locally generated and does not depend on The AD844 provides very clean pulse response at high feedback. This is very different from the virtual ground of a noninverting gains. Figure 32 shows a typical configuration conventional operational amplifier used in the current summing providing a gain of 100 with high input resistance. The feedback mode, which is essentially an open circuit until the loop settles. resistor is kept as low as practicable to maximize bandwidth, In the AD844, transient current at the input does not cause and a peaking capacitor (CPK) can optionally be added to voltage spikes at the summing node while the amplifier is further extend the bandwidth. Figure 33 shows the small signal settling. Furthermore, all of the transient current is delivered response with CPK = 3 nF, RL = 500 Ω, and supply voltages of to the slewing (TZ) node (Pin 5) via a short signal path (the either ±5 V or ±15 V. Gain bandwidth products of up to grounded base stages and the wideband current mirrors). 900 MHz can be achieved in this way. The current available to charge the capacitance (about 4.5 pF) at The offset voltage of the AD844 is laser trimmed to the 50 μV the TZ node is always proportional to the input error current, level and exhibits very low drift. In practice, there is an and the slew rate limitations associated with the large signal additional offset term due to the bias current at the inverting response of the op amps do not occur. For this reason, the rise input (IBN), which flows in the feedback resistor (R1). This can and fall times are almost independent of signal level. In practice, optionally be nulled by the trimming potentiometer shown in the input current eventually causes the mirrors to saturate. Figure 32. When using ±15 V supplies, this occurs at about 10 mA (or
+VS
±2200 V/μs). Because signal currents are rarely this large, classical slew rate limitations are absent.
4.7Ω OFFSET
This inherent advantage is lost if the voltage follower used to
TRIM
buffer the output has slew rate limitations. The AD844 is
CPK R1
designed to avoid this problem, and as a result, the output
3nF 20kΩ 499Ω
buffer exhibits a clean large signal transient response, free from
1
anomalous effects arising from internal saturation.
8 0.22µF 2 RESPONSE AS A NONINVERTING AMPLIFIER R2 7 4.99Ω
Because current feedback amplifiers are asymmetrical with
AD844 6
regard to their two inputs, performance differs markedly in
VIN 3 RL
noninverting and inverting modes. In noninverting modes, the
4 0.22µF
large signal high speed behavior of the AD844 deteriorates at
4.7Ω
low gains because the biasing circuitry for the input system (not 32 0 shown in Figure 31) is not designed to provide high input
–V
97-
S
008 voltage slew rates. Figure 32. Noninverting Amplifier Gain = 100, Optional Offset Trim Is Shown However, good results can be obtained with some care. The
46
noninverting input does not tolerate a large transient input; it must be kept below ±1 V for best results. Consequently, this
VS = ±15V 40
mode is better suited to high gain applications (greater than ×10). Figure 23 shows a noninverting amplifier with a gain of 10
VS = ±5V
and a bandwidth of 30 MHz. The transient response is shown in
) 34 B d
Figure 26 and Figure 27. To increase the bandwidth at higher
( IN
gains, a capacitor can be added across R2 whose value is
GA 28
approximately (R1/R2) × Ct.
22 16 100k 1M 10M 20M
0-047
FREQUENCY (Hz)
89 00 Figure 33. AC Response for Gain = 100, Configuration Shown in Figure 32 Rev. G | Page 14 of 20 Document Outline FEATURES APPLICATIONS FUNCTIONAL BLOCK DIAGRAMS GENERAL DESCRIPTION PRODUCT HIGHLIGHTS TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS METALLIZATION PHOTOGRAPH ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS INVERTING GAIN-OF-1 AC CHARACTERISTICS INVERTING GAIN-OF-10 AC CHARACTERISTICS INVERTING GAIN-OF-10 PULSE RESPONSE NONINVERTING GAIN-OF-10 AC CHARACTERISTICS UNDERSTANDING THE AD844 OPEN-LOOP BEHAVIOR RESPONSE AS AN INVERTING AMPLIFIER RESPONSE AS AN I-V CONVERTER CIRCUIT DESCRIPTION OF THE AD844 RESPONSE AS A NONINVERTING AMPLIFIER NONINVERTING GAIN OF 100 USING THE AD844 BOARD LAYOUT INPUT IMPEDANCE DRIVING LARGE CAPACITIVE LOADS SETTLING TIME DC ERROR CALCULATION NOISE VIDEO CABLE DRIVER USING ±5 V SUPPLIES HIGH SPEED DAC BUFFER 20 MHZ VARIABLE GAIN AMPLIFIER OUTLINE DIMENSIONS ORDERING GUIDE