Datasheet AD8024 (Analog Devices) - 9

AD8024 General Driving Capacitive Loads
The AD8024 is a wide bandwidth, quad video amplifier. It offers a When used in combination with the appropriate feedback resistor, high level of performance on 16 mA total quiescent supply cur- the AD8024 will drive any load capacitance without oscillation. rent for closed-loop gains of ± 1 or greater. In accordance with the general rule for current feedback ampli- Bandwidth up to 380 MHz, low differential gain and phase errors, fiers, increased load capacitance requires the use of a higher and high output current make the AD8024 an efficient video feedback resistor for stable operation. amplifier. Due to the high open-loop transresistance and low inverting The AD8024’s wide phase margin and high output current make it input current of the AD8024, large feedback resistors do not an excellent choice when driving any capacitive load. create large closed-loop gain errors. In addition, the high output current allows rapid voltage slewing on large load capacitors.
Choice of Feedback Resistor
Because it is a current feedback amplifier, the closed-loop For wide bandwidth and clean pulse response, an additional bandwidth of the AD8024 may be customized with the feed- small series output resistor of about 10 Ω is recommended. back resistor.
RF
A larger feedback resistor reduces peaking and increases the
+V 1.0 F S
phase margin at the expense of reduced bandwidth. A smaller
0.1 F
feedback resistor increases bandwidth at the expense of increased
RG –
peaking and reduced phase margin.
RS AD8024 V 1.0 F O
The closed-loop bandwidth is affected by attenuation due to the
V + CL IN
finite output resistance. The open-loop output resistance of ≈6 Ω
R 0.1 F T
reduces the bandwidth somewhat when driving load resistors less
–VS
than ≈150 Ω. The bandwidth will be ≈10% greater for load resis- Figure 3. Circuit for Driving a Capacitive Load tance above a few hundred ohms. The value of the feedback resistor is not critical unless maintaining the widest or flattest frequency response is desired. Table I shows
1V 20nS
the bandwidth at different supply voltages for some useful closed- loop gains when driving a 150 Ω load. The recommended resistors
VIN
are for the widest bandwidth with less than 2 dB peaking.
Table I. –3 dB Bandwidth vs. Closed-Loop Gain Resistor, RL = 150 V V OUT S – Volts Gain RF – BW – MHz
±7.5 +1 5000 350 +2 750 275
2V
+10 400 105 –1 750 165 Figure 4. Pulse Response Driving a Large Load ±12 +1 8000 380 Capacitance, CL = 300 pF, G = +3, RFB = 2.32 kΩ, +10 215 150 R –1 750 95 S = 10.5 Ω, RL = 1 kΩ, VS = ±7.5 V ±2.5 +2 1125 125 REV. C –9–