Datasheet AD8023 (Analog Devices) - 10

AD8023 +1
G
GAIN
ACL
dB 0 –
1 + SC ( R + Gn rin ) T F
–1
where: CT = transcapacitance 1 pF
VS = 7.5V
R
–2
F = feedback resistor G = ideal closed loop gain
–3 VS = 2.5V PHASE
 
–4 0
Gn = 1 + RF R   = noise gain G
–5
rin = inverting input resistance 150 Ω
Degrees – –6 –90
ACL = closed loop gain
G = –10 –7 RL = 150 VS = 2.5V
The –3 dB bandwidth is determined from this model as:
CLOSED-LOOP GAIN (NORMALIZED) –8 –180
f
PHASE SHIFT
3 1
–9
2 π C (R + Gn rin ) T F
1 10 100 500 FREQUENCY – MHz
This model will predict –3 dB bandwidth to within about 10% to 15% of the correct value when the load is 150 Ω and Figure 30. Closed-Loop Gain and Phase vs. Frequency, VS = ±7.5 V. For lower supply voltages there will be a slight G = –10, RL = 150 Ω decrease in bandwidth. The model is not accurate enough to
General
predict either the phase behavior or the frequency response The AD8023 is a wide bandwidth, triple video amplifier that peaking of the AD8023. offers a high level of performance on less than 9.0 mA per It should be noted that the bandwidth is affected by attenuation amplifier of quiescent supply current. The AD8023 achieves due to the finite input resistance. Also, the open-loop output bandwidth in excess of 200 MHz, with low differential gain and resistance of about 6 Ω reduces the bandwidth somewhat when phase errors and high output current making it an efficient video driving load resistors less than about 150 Ω. (Bandwidths will amplifier. be about 10% greater for load resistances above a couple The AD8023’s wide phase margin coupled with a high output hundred ohms.) short circuit current make it an excellent choice when driving any capacitive load up to 300 pF.
Table I. –3 dB Bandwidth vs. Closed-Loop Gain and Feedback Resistor, RL = 150 (SOIC)
It is designed to offer outstanding functionality and performance at closed-loop inverting or noninverting gains of one or greater.
VS – Volts Gain RF – Ohms BW – MHz Choice of Feedback and Gain Resistors
±7.5 +1 2000 460 +2 750 240 Because it is a current feedback amplifier, the closed-loop band- +10 300 50 width of the AD8023 may be customized using different values –1 750 150 of the feedback resistor. Table I shows typical bandwidths at –10 250 60 different supply voltages for some useful closed-loop gains when ±2.5 +1 2000 250 driving a load of 150 Ω. +2 1000 90 The choice of feedback resistor is not critical unless it is desired +10 300 30 to maintain the widest, flattest frequency response. The resistors –1 750 95 recommended in the table (chip resistors) are those that will –10 250 50 result in the widest 0.1 dB bandwidth without peaking. In
Driving Capacitive Loads
applications requiring the best control of bandwidth, 1% When used in combination with the appropriate feedback resistors are adequate. Resistor values and widest bandwidth resistor, the AD8023 will drive any load capacitance without figures are shown. Wider bandwidths than those in the table can oscillation. The general rule for current feedback amplifiers is be attained by reducing the magnitude of the feedback resistor that the higher the load capacitance, the higher the feedback (at the expense of increased peaking), while peaking can be resistor required for stable operation. Due to the high open-loop reduced by increasing the magnitude of the feedback resistor. transresistance and low inverting input current of the AD8023, Increasing the feedback resistor is especially useful when driving the use of a large feedback resistor does not result in large closed- large capacitive loads as it will increase the phase margin of the loop gain errors. Additionally, its high output short circuit current closed-loop circuit. (Refer to the Driving Capacitive Loads makes possible rapid voltage slewing on large load capacitors. section for more information.) For the best combination of wide bandwidth and clean pulse To estimate the –3 dB bandwidth for closed-loop gains of 2 or response, a small output series resistor is also recommended. greater, for feedback resistors not listed in the following table, Table II contains values of feedback and series resistors which the following single pole model for the AD8023 may be used: result in the best pulse responses. Figure 28 shows the AD8023 driving a 300 pF capacitor through a large voltage step with virtually no overshoot. (In this case, the large and small signal pulse responses are quite similar in appearance.) REV. A –9–