OPA600BM BURR-BROWN [Burr-Brown Corporation], OPA600BM Datasheet - Page 6

• Manufacturer Part Number: OPA600BM
• Description: Fast-Settling Wideband OPERATIONAL AMPLIFIER
• Manufacturer: BURR-BROWN [Burr-Brown Corporation]

COMPENSATION
The OPA600 uses external frequency compensation so that
the user may optimize the bandwidth or settling time for his
particular application. Several performance curves aid in the
selection of the correct compensation’s capacitance value.
The Bode plot shows amplitude and phase versus frequency
for several values of compensation. A related curve shows
the recommended compensation capacitance versus closed-
loop gain.
Figure 1 shows a recommended circuit schematic. Compo-
nent values and compensation for amplifiers with several
different closed-loop gains are shown. This circuit will yield
the specified settling time. Because each device is unique
and slightly different, as is each user’s circuit, optimum
settling time will be achieved by individually compensating
each device in its own circuit, if desired. A 10% to 20%
improvement in settling time has been experienced from the
values indicated in the Electrical Specifications table.
FIGURE 1. Recommended Amplifier Circuits and Frequency
The primary compensation capacitors are C
Figure 1). They are connected between pins 4 and 5 and
between pins 11 and 14. Both C
value. As Figure 1 and the performance curves show, larger
closed-loop configurations require less capacitance, and
improved gain-bandwidth product can be realized. Note that
no compensation capacitor is required for closed-loop gains
equal to or above 100V/V. If upon initial application the
user’s circuit is unstable, and remains so after checking for
proper bypassing, grounding, etc., it may be necessary to
increase the compensation slightly to eliminate oscillations.
Do not over compensate. It should not be necessary to
increase C
necessary to individually optimize C
performance.
Closed
–1000
Loop
Gain
–100
–10
+1
–1
R
51
5
open
620
100
100
100
1
R
Compensation.
1
and C
R
1
3.3k
3.3k
100
620
R
1k
2
2
beyond 10pF to 15pF. It may also be
1µF
short
short
short
3.3k
3.3k
C
R
3
3
+
R
open
open
open
+
3.2k
116
R
2
13
16
4
3
1
1
–
+
OPA600
and C
C
12
6
6.8
3.3
1
+15VDC
1
0
0
–
–15VDC
C
11
C
4
C
1
2
1
2
and C
2
5
14
2.2
4.7
should be the same
C
0
1
0
3
8
R
2
1
4
for improved
4.7
C
R
0
0
0
0
and C
4
3
C
4
100
100
100
R
56
–
5
2
(see
The flat high frequency response of the OPA600 is pre-
served and high frequency peaking is minimized by connect-
ing a small capacitor in parallel with the feedback resistor
(see Figure 1). This capacitor compensates for the closed-
loop, high frequency, transfer function zero that results from
the time constant formed by the input capacitance of the
amplifier, typically 2pF, and the input and feedback resis-
tors. The selected compensation capacitor may be a trimmer,
a fixed capacitor, or a planned PC board capacitance. The
capacitance value is strongly dependent on circuit layout and
closed-loop gain. It will typically be 2pF for a clean layout
using low resistances (1k ) and up to 10pF for circuits using
larger resistances. Using small resistor values will preserve
the phase margin and avoid peaking by keeping the break
frequency of this zero sufficiently high. When high closed-
loop gains are required, a three-resistor attenuator is recom-
mended to avoid using a large value resistor with its long
time constant.
CAPACITIVE LOADS
The OPA600 will drive large capacitive loads (up to 100pF)
when properly compensated and settling times of under
150ns are achievable. The effect of a capacitive load is to
decrease the phase margin of the amplifier, which may cause
high frequency peaking or oscillations. A solution is to
increase the compensation capacitance, somewhat slowing
the amplifier’s ability to respond. The recommended com-
pensation capacitance value as a function of load capaci-
tance is shown in Figure 2. (Use two capacitors, each with
the value indicated.) Alternately, without increasing the
OPA600’s compensation capacitance, the capacitive load
may be buffered by connecting a small resistance, usually
5
For very-large capacitive loads, greater than 100pF, it will
be necessary to use doublet compensation. Refer to Figure 3
and discussion on slew rate. This places the dominant pole
at the input stage. Settling time will be approximately 50%
slower; slew rate should increase. Load capacitance should
be minimized for optimum high frequency performance.
Because of its large output capability, the OPA600 is par-
ticularly well suited for driving loads via coaxial cables.
Note that the capacitance of coaxial cable (29pF/foot of
length for RG-58) will not load the amplifier when the
coaxial cable or transmission line is terminated in its char-
acteristic impedance.
SETTLING TIME
Settling time is defined as the total time required, from the
input signal step, for the output to settle within the specified
error band around the final value. This error band is ex-
pressed as a percentage of the magnitude of the output
transition, a 10V step.
Settling time is a complete dynamic measure of the OPA600’s
total performance. It includes the slew rate time, a large
signal dynamic parameter, and the time to accurately reach
the final value, a small signal parameter that is a function of
bandwidth and open-loop gain. Performance curves show
to 50 , in series with the Output, pin 8.