OP-44_15 Datasheet, PDF (9/9 Pages) Analog Devices – HIGH SPEED PRECISION OPERATIONAL AMPLIFIER

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!PMI) OP-44 HIGH-SPEED, PRECISION OPERATIONAL AMPLIFIER
FIGURE 9: Programmable Baseline Restorer
+15V
R2
2k11
R1
V'N
2k11
VPROGRAM
VOUT
RpROGRAM
15k11
VREF
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-15V
O_{Baseline restoration is another useful technique for correcting}
B^{errors introduced by amplifier drift, or by electromagnetic}
pickup. High-impedance sources, such as a human body, are
S _{notorious for large DC drifts. In many cases, where pulse or AC}
measurements are being made, and the pulse height above a
O _{nominal DC line contains the important information.}
. ^{L While a simple high-pass filter may be adequate for some}
situations, the baseline restorer shown in Figure 9 allows a wide
E _{degree of flexibility for analog adaptive filtering techniques, and}
T offers some benefits not available with a frequency-domain
E ^filter.
FIGURE 10: Baseline Restorer Response
The baseline restorer behaves as a nonlinear filter, acting upon
the slew rate of the input signal rather than its frequency. Its
output will restore the base of the pulses to an arbitrary level, set
by VREF.The slew rate cutoff of the filter is set by the current
flowing through 01, which is in turn set by VPROGRAMV.REFand
VPROGRAMmay be controlled by a voltage-output DACsuch as
the PM-7226. Ifcurrent programming is desired, RpROGRAMmay
be removed and replaced by a current-source, such as a bipolar
DAC.
To understand the circuit's operation, assume that capacitor C
has charged to the DC baseline. If the output swings above the
baseline, IC2 swings low, reverse biasing diode D2. D1 is pulled
low, and forward biases. A current (12 - '1) discharges the
capacitor until equilibrium is restored. Ifthe output drops below
the baseline, IC2 swings high, and D2 becomes forward biased.
12is supplied by the output of IC2 while 11charges C until the
baseline is restored. The rate of restoration depends upon the
current available to charge or discharge C.
For symmetric operation, with the same restoration rate for
positive or negative excursions from the baseline, 12must be
twice '1, This provides an equal current for charging and
discharging the capacitor. 11 is set by the current flowing
through 01 in the MAT-O4.An identical current flows through
each transistor. The MAT-O3matched PNP pair, 05 and 06, act
as a current mirror to reflect the current through 02 (11).03 and
04 create '2, which is twice 11.11may be set anywhere between a
few nanoamps to several mA. Higher currents will result in
rejection of faster-slewing signals, while lower currents will
allow passage of slower signals.
The OP-44 is configured for a gain of -1, but gain is adjustable
by R1 and R2, and is simply -(R2/R1). OP-44 stability is main-
tained by the dominant pole introduced by C.
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10/87, Rev. 81