TLE2662 Datasheet, PDF (29/40 Pages) Texas Instruments – DUAL uPOWER JFET-INPUT OPERATIONAL AMPLIFIER WITH SWITCHED-CAPACITOR VOLTAGE CONVERTER

TLE2662
DUAL µPOWER JFET-INPUT OPERATIONAL AMPLIFIER
WITH SWITCHED-CAPACITOR VOLTAGE CONVERTER
SLOS118B – DECEMBER 1992 – REVISED AUGUST 1994
APPLICATION INFORMATION
precision measurement techniques
In systems where the amplifier outputs are being sampled by an analog-to-digital converter (ADC), the
switched-capacitor network can be temporarily disabled by applying a voltage of less then 0.45 V to FB/SD. This
is easily accomplished using any open-collector gate (shown by dashed lines in Figure 49). When disabled, the
internal switches are set to dump any remaining charge onto COUT. The voltage at SCOUT decays to zero at
a rate dependent on both the size of COUT and loading. During this time, the amplifier’s outputs are free of any
switching-induced ripple and noise. Figure 56 shows the relationship of the output voltage decay time to the size
of the output storage capacitor when one channel of the amplifier is driving a 100-Ω load to ground. SCOUT
rises again when the external gate is turned off (see Figure 57).
OFF-STATE VOLTAGE DECAY AT OUTPUT
vs
TIME
6
VCC+ = 5 V
VCC _– = SCOUT
4 CIN = 100 µF
RL = 100 Ω
VID = – 100 mV
2
TURN-ON VOLTAGE RISE AT OUTPUT
vs
TIME
6
VCC+ = 5 V
VCC _– = SCOUT
4 CIN = 100 µF
RL = 100 Ω
VID = – 100 mV
2
0
COUT = 22 µF
COUT = 100 µF
–2
COUT = 220 µF
–4
–6
0
10 20 30 40 50 60 70 80
t – Time – ms
Figure 56
0
COUT = 100 µF
–2
COUT = 220 µF
–4
–6
0
COUT = 22 µF
10 20 30 40 50 60 70 80
t – Time – ms
Figure 57
The amplifier’s negative input common-mode voltage limit (V_{ICR –}) is specified as an offset from the negative
rail. Care should be taken to ensure that the input signal does not violate this limit as SCOUT decays. The
negative output voltage swing is similarly affected by the gradual loss of the negative rail.
This application takes advantage of the otherwise unused SCREF output of the switched-capacitor block to bias
one amplifier to 2.5 V. This is especially useful when the amplifier is followed by an ADC, keeping the signal
centered in the middle of the converter dynamic range. Other biasing methods may be necessary in precision
systems.
In Figure 58, SCREF , R1, and R2 are used to generate a feedback voltage to the TLE2662 error amplifier. This
voltage, fed into FB/SD, is used to regulate the voltage at SCOUT. When used this way, there is higher voltage
loss ( SCIN – |SCOUT| ) associated with the regulation. For example, the inverter generates an unregulated
voltage of approximately – 4.5 V from a positive 5-V source; it can achieve a regulated output voltage of only
about – 3.5 V. Though this reduces the amplifier input and output dynamic range, both V_{ICR –} and V_OL still extend
to below ground.
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