Datasheet AD827 (Analog Devices) - 9

AD827 A TWO-CHIP VOLTAGE-CONTROLLED AMPLIFIER
between the CH1 output and Z1, the other between the CH1
(VCA) WITH EXPONENTIAL RESPONSE
output and W1. Likewise, in the CH2 multiplier, one of the Voltage-controlled amplifiers are often used as building blocks feedback resistors is connected between CH2 and Z2 and the in automatic gain control systems. Figure 25 shows a two-chip other is connected between CH2 and Z2. In Figure 25, Z1 and VCA built using the AD827 and the AD539, a dual, current- W1 are tied together, as are Z2 and W2, providing a 3 kΩ output multiplier. As configured, the circuit has its two feedback resistor for the op amp. The 2 pF capacitors connected between the AD539’s W1 and CH1 and W2 and CH2 pins are in parallel with the feedback resistors and thus reduce peaking in the VCA’s frequency response. Increasing the values of C3 and C4 can further reduce the peaking at the expense of reduced bandwidth. The 1.25 mA full-scale output current of the AD539 and the 3 kΩ feedback resistor set the full-scale output voltage of each multiplier at 3.25 V p-p. Current limiting in the AD827 (typically 30 mA) limits the out- put voltage in this application to about 3 V p-p across a 100 Ω load. Driving a 50 Ω reverse-terminated load divides this value by two, limiting the maximum signal delivered to a 50 Ω load to about 1.5 V p-p, which suffices for video signal levels. The dynamic range of this circuit is approximately 55 dB and is primarily limited by feedthrough at low input levels and by the maximum output voltage at high levels. Figure 25. A Wide Range Voltage-Controlled
Guidelines for Grounding and Bypassing
Amplifier Circuit When designing practical high frequency circuits using the AD827, some special precautions are in order. Both short interconnection leads and a large ground plane are needed whenever possible to multipliers connected in series. They could also be placed in provide low resistance, low inductance circuit paths. One should parallel with an increase in bandwidth and a reduction in gain. remember to minimize the effects of capacitive coupling The gain of the circuit is controlled by VX, which can range between circuits. Furthermore, IC sockets should be avoided. from 0 to 3 V dc. Measurements show that this circuit easily Feedback resistors should be of a low enough value that the supplies 2 V p-p into a 100 Ω load while operating from ± 5 V time constant formed with stray circuit capacitances at the supplies. The overall bandwidth of the circuit is approximately amplifier summing junction will not limit circuit performance. 7 MHz with 0.5 dB of peaking. As a rule of thumb, use feedback resistor values that are less Each half of the AD827 serves as an I/V converter and converts than 5 kΩ. If a larger resistor value is necessary, a small (<10 pF) the output current of one of the two multipliers in the AD539 feedback capacitor in parallel with the feedback resistor may be into an output voltage. Each of the AD539’s two multipliers used. The use of 0.1 µF ceramic disc capacitors is recommended contains two internal 6 kΩ feedback resistors; one is connected for bypassing the op amp’s power supply leads. –8– REV. C Document Outline FEATURES CONNECTION DIAGRAMS PRODUCT DESCRIPTION APPLICATION HIGHLIGHTS SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ORDERING GUIDE METALLIZATION PHOTOGRAPH Typical Performance Characteristics INPUT PROTECTION PRECAUTIONS VIDEO LINE DRIVER A HIGH SPEED THREE OP AMP INSTRUMENTATION AMPLIFIER CIRCUIT A TWO-CHIP VOLTAGE-CONTROLLED AMPLIFIER (VCA) WITH EXPONENTIAL RESPONSE Guidelines for Grounding and Bypassing OUTLINE DIMENSIONS Revision History