Datasheet OP285 (Analog Devices) - 10

OP285
thereby reducing phase error dramatically. This is shown in
A Low Noise, High Speed Instrumentation Amplifier
Figure 13 where the 10x composite amplifier’s phase response A high speed, low noise instrumentation amplifier, constructed exhibits less than 1.5° phase shift through 500 kHz. On the other with a single OP285, is illustrated in Figure 15. The circuit exhibits hand, the single gain stage amplifier exhibits 25° of phase shift less than 1.2 µV p-p noise (RTI) in the 0.1 Hz to 10 Hz band over the same frequency range. An additional benefit of the low and an input noise voltage spectral density of 9 nV/√Hz (1 kHz) phase error configuration is constant group delay, by virtue of at a gain of 1000. The gain of the amplifier is easily set by RG constant phase shift at all frequencies below 500 kHz. Although according to the formula: this technique is valid for minimum circuit gains of 10, actual closed-loop magnitude response must be optimized for the VOUT 9 9 . 8 kΩ = + 2 amplifier chosen. VIN RG The advantages of a two op amp instrumentation amplifier based on a dual op amp is that the errors in the individual am-
LOW PHASE ERROR 0 AMPLIFIER RESPONSE
plifiers tend to cancel one another. For example, the circuit’s input offset voltage is determined by the input offset voltage
–5
matching of the OP285, which is typically less than 250 µV.
–10 –15 + SINGLE STAGE 5 AMPLIFIER RESPONSE VIN 3 Degrees 7 –20 – – 1 A2 V 6 OUT 2 A1 –25 AC CMRR TRIM R3 4.99k PHASE –30 R4 R2 C1 4.99k 4.99 –35 5pF–40pF RG A1, A2 = 1/2 OP285 R1 –40 9.98k 4.99k GAIN = +2 DC CMRR TRIM RQ –45 GAIN R 10k 100k 1M 10M G( ) P1 START 10,000.000Hz STOP 10,000,000.000Hz 2 OPEN 500 10 1.24k 100 102
Figure 13. Phase Error Comparison
1000 10
For a more detailed treatment on the design of low phase error Figure 15. A High-Speed Instrumentation Amplifier amplifiers, see Application Note AN-107. Common-mode rejection of the circuit is limited by the matching
Fast Current Pump
of resistors R1 to R4. For good common-mode rejection, these A fast, 30 mA current source, illustrated in Figure 14, takes resistors ought to be matched to better than 1%. The circuit was advantage of the OP285’s speed and high output current drive. constructed with 1% resistors and included potentiometer P1 This is a variation of the Howland current source where a sec- for trimming the CMRR and a capacitor C1 for trimming the ond amplifier, A2, is used to increase load current accuracy and CMRR. With these two trims, the circuit’s common-mode output voltage compliance. With supply voltages of ± 15 V, the rejection was better than 95 dB at 60 Hz and better than 65 dB output voltage compliance of the current pump is ± 8 V. To at 10 kHz. For the best common-mode rejection performance, keep the output resistance in the MΩ range requires that 0.1% use a matched (better than 0.1%) thin-film resistor network for or better resistors be used in the circuit. The gain of the current R1 through R4 and use the variable capacitor to optimize the pump can be easily changed according to the equations shown circuit’s CMR. in the diagram. The instrumentation amplifier exhibits very wide small- and
R1 R2
large-signal bandwidths regardless of the gain setting, as shown
2k 2k VIN1
in the table. Because of its low noise, wide gain-bandwidth
2 R5
product, and high slew rate, the OP285 is ideally suited for high
R3 1 50 A1 2k 3
speed signal conditioning applications.
VIN2 R4 5 VIN2 – V IN1 VIN = 2k IOUT = 7 R5 R5 A2 Circuit RG Circuit Bandwidth 6 IOUT = (MAX) = 30mA A1, A2 = 1/2 OP285 Gain ( ) VOUT = 100 mV p-p VOUT = 20 V p-p R2 GAIN = , R4 = R2, R3 = R1 R1
2 Open 5 MHz 780 kHz Figure 14. A Fast Current Pump 10 1.24 k 1 MHz 460 kHz 100 102 90 kHz 85 kHz 1000 10 10 kHz 10 kHz –10– REV. C