REV. A
OP285
–10–
thereby reducing phase error dramatically. This is shown in
Figure 13 where the 10x composite amplifier’s phase response
exhibits less than 1.5° phase shift through 500 kHz. On the other
hand, the single gain stage amplifier exhibits 25° of phase shift
over the same frequency range. An additional benefit of the low
phase error configuration is constant group delay, by virtue of
constant phase shift at all frequencies below 500 kHz. Although
this technique is valid for minimum circuit gains of 10, actual
closed-loop magnitude response must be optimized for the
amplifier chosen.
–20
–45
10k 100k 10M1M
–25
–30
–35
–40
–15
–10
–5
0
START 10,000.000Hz STOP 10,000,000.000Hz
PHASE – Degrees
SINGLE STAGE
AMPLIFIER RESPONSE
LOW PHASE ERROR
AMPLIFIER RESPONSE
Figure 13. Phase Error Comparison
For a more detailed treatment on the design of low phase error
amplifiers, see Application Note AN-107.
Fast Current Pump
A fast, 30 mA current source, illustrated in Figure 14, takes
advantage of the OP285’s speed and high output current drive.
This is a variation of the Howland current source where a sec-
ond amplifier, A2, is used to increase load current accuracy and
output voltage compliance. With supply voltages of ±15 V, the
output voltage compliance of the current pump is ±8 V. To
keep the output resistance in the MΩ range requires that 0.1%
or better resistors be used in the circuit. The gain of the current
pump can be easily changed according to the equations shown
in the diagram.
1
2
3
A1
5
6
7
V
IN1
V
IN2
A2
A1, A2 = 1/2 OP285
R2
R1
GAIN = , R4 = R2, R3 = R1
R1
2k
R2
2k
R5
50
R3
2k
R4
2k
I
OUT
=
V
IN2
– V
IN1
R5
V
IN
R5
=
I
OUT
= (MAX) = 30mA
Figure 14. A Fast Current Pump
A Low Noise, High Speed Instrumentation Amplifier
A high speed, low noise instrumentation amplifier, constructed
with a single OP285, is illustrated in Figure 15. The circuit exhibits
less than 1.2 µV p-p noise (RTI) in the 0.1 Hz to 10 Hz band
and an input noise voltage spectral density of 9 nV/√Hz (1 kHz)
at a gain of 1000. The gain of the amplifier is easily set by R
G
according to the formula:
V
V
k
R
OUT
IN G
=+
998
2
. Ω
The advantages of a two op amp instrumentation amplifier
based on a dual op amp is that the errors in the individual am-
plifiers tend to cancel one another. For example, the circuit’s
input offset voltage is determined by the input offset voltage
matching of the OP285, which is typically less than 250 µV.
1
2
3
A2
A1
5
6
7
V
IN
A1, A2 = 1/2 OP285
R
Q
9.98k
+2
GAIN =
R
G
()
OPEN
1.24k
102
10
2
10
100
1000
GAIN
R1
4.99k
P1
500
DC CMRR TRIM
AC CMRR TRIM
C1
5pF–40pF
+
–
R
G
R2
4.99
R3
4.99k
R4
4.99k
V
OUT
Figure 15. A High-Speed Instrumentation Amplifier
Common-mode rejection of the circuit is limited by the matching
of resistors R1 to R4. For good common-mode rejection, these
resistors ought to be matched to better than 1%. The circuit was
constructed with 1% resistors and included potentiometer P1
for trimming the CMRR and a capacitor C1 for trimming the
CMRR. With these two trims, the circuit’s common-mode
rejection was better than 95 dB at 60 Hz and better than 65 dB
at 10 kHz. For the best common-mode rejection performance,
use a matched (better than 0.1%) thin-film resistor network for
R1 through R4 and use the variable capacitor to optimize the
circuit’s CMR.
The instrumentation amplifier exhibits very wide small- and
large-signal bandwidths regardless of the gain setting, as shown
in the table. Because of its low noise, wide gain-bandwidth
product, and high slew rate, the OP285 is ideally suited for high
speed signal conditioning applications.
Circuit R
G
Circuit Bandwidth
Gain () V
OUT
= 100 mV p-p V
OUT
= 20 V p-p
2 Open 5 MHz 780 kHz
10 1.24 k 1 MHz 460 kHz
100 102 90 kHz 85 kHz
1000 10 10 kHz 10 kHz
