AD8057ART-EBZ View Datasheet(PDF) - Analog Devices
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AD8057ART-EBZ Datasheet PDF : 17 Pages
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Data Sheet
APPLICATIONS INFORMATION
DRIVING CAPACITIVE LOADS
When driving a capacitive load, most op amps exhibit overshoot in
their pulse response. Figure 43 shows the relationship between
the capacitive load that results in 30% overshoot and the closed-
loop gain of an AD8058. It can be seen that, under the gain = +2
condition, the device is stable with capacitive loads of up to 69 pF.
In general, to minimize peaking or to ensure device stability for
larger values of capacitive loads, a small series resistor (RS) can
be added between the op amp output and the load capacitor
(CL) as shown in Figure 44.
For the setup shown in Figure 44, the relationship between RS
and CL was empirically derived and is shown in Table 4.
500
400
300
200
RS = 2.4Ω
100
RS = 0Ω
0
1
2
3
4
5
CLOSED-LOOP GAIN
Figure 43. Capacitive Load Drive vs. Closed-Loop Gain
RF
+2.5V
RG
VIN = 200mV p-p
0.1µF
10µF
AD8058
RS FET PROBE
VOUT
CL
0.1µF 10µF
–2.5V
Figure 44. Capacitive Load Drive Circuit
AD8057/AD8058
Table 4. Recommended Value for Resistors RS, RF, RG vs.
Capacitive Load, CL, Which Results in 30% Overshoot
Gain RF
RG
CL (RS = 0 Ω) CL (RS = 2.4 Ω)
1
100 Ω
11 pF
13 pF
2
100 Ω 100 Ω 51 pF
69 pF
3
100 Ω 50 Ω 104 pF
153 pF
4
100 Ω 33.2 Ω 186 pF
270 pF
5
100 Ω 25 Ω 245 pF
500 pF
10
100 Ω 11 Ω 870 pF
1580 pF
200mV
100mV
+OVERSHOOT
29.0%
–100mV
–200mV
100mV/DIV
50ns/DIV
Figure 45. Typical Pulse Response with CL = 65 pF, Gain = +2, and VS = ±2.5
VIDEO FILTER
Some composite video signals that are derived from a digital
source contain some clock feedthrough that can cause problems
with downstream circuitry. This clock feedthrough is usually at
27 MHz, which is a standard clock frequency for both NTSC
and PAL video systems. A filter that passes the video band and
rejects frequencies at 27 MHz can be used to remove these fre-
quencies from the video signal.
Figure 46 shows a circuit that uses an AD8057 to create a single
5 V supply, 3-pole Sallen-Key filter. This circuit uses a single RC
pole in front of a standard 2-pole active section. To shift the dc
operating point to midsupply, ac coupling is provided by R4, R5,
and C4.
C2
680pF
RF
1kΩ
+5V
R1
200Ω
R2
499Ω
C1
100pF
R3
49.9Ω
+5V
C4 R4
0.1µF 10kΩ
C3
36pF
R5
10kΩ
7
2
0.1µF
AD8057 6
3
4
+
10µF
Figure 46. Low-Pass Filter for Video
Rev. D | Page 13 of 16
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