OPA689 Burr-Brown, OPA689 Datasheet - Page 16

• Manufacturer Part Number: OPA689
• Description: Wideband / High Gain VOLTAGE LIMITING AMPLIFIER
• Manufacturer: Burr-Brown

FREQUENCY RESPONSE COMPENSATION
The OPA689 is internally compensated to be stable at a gain
of +4, and has a nominal phase margin of 60 at a gain of +6.
Phase margin and peaking improve at higher gains. Recall
that an inverting gain of –5 is equivalent to a gain of +6 for
bandwidth purposes (i.e., noise gain = 6).
Standard external compensation techniques work with this
device. For example, in the inverting configuration, the
bandwidth may be limited without modifying the inverting
gain by placing a series RC network to ground on the
inverting node. This has the effect of increasing the noise
gain at high frequencies, which limits the bandwidth.
To maintain a large bandwidth at high gains, cascade several
op amps.
In applications where a large feedback resistor is required,
such as photodiode transimpedance amplifier, the parasitic
capacitance from the inverting input to ground causes peak-
ing or oscillations. To compensate for this effect, connect a
small capacitor in parallel with the feedback resistor. The
bandwidth will be limited by the pole that the feedback
resistor and this capacitor create. In other high gain applica-
tions, use a three resistor “Tee” network to reduce the RC
time constants set by the parasitic capacitances. Be careful
to not increase the noise generated by this feedback network
too much.
PULSE SETTLING TIME
The OPA689 is capable of an extremely fast settling time in
response to a pulse input. Frequency response flatness and
phase linearity are needed to obtain the best settling times.
For capacitive loads, such as an A/D converter, use the
®
OPA689
16
recommended R
tremely fine scale settling (0.01%) requires close attention to
ground return current in the supply decoupling capacitors.
The pulse settling characteristics when recovering from
overdrive are very good.
DISTORTION
The OPA689’s distortion performance is specified for a
500
smaller resistance will increase the distortion as illustrated in
Figure 11. Remember to include the feedback network in the
load resistance calculations.
FIGURE 11. 5MHz Harmonic Distortion vs Load Resistance.
–40
–45
–50
–55
–60
–65
–70
–75
–80
–85
–90
load, such as an A/D converter. Driving loads with
50
HARMONIC DISTORTION vs LOAD RESISTANCE
S
100
in the R
Load Resistance ( )
HD3
HD2
S
vs Capacitive Load plot. Ex-
V
f
1
O
= 5MHz
= 2Vp-p
1000