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TC7652 Datasheet PDF : 6 Pages
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LOW NOISE, CHOPPER-STABILIZED
OPERATIONAL AMPLIFIER
1
TC7652
Capacitor Connection
Connect the null-storage capacitors to the CA and CB
pins with a common connection to the CRET pin (14-pin
TC7652) or to VSS (8-pin TC7652). When connecting to VSS,
avoid injecting load current IR drops into the capacitive
circuitry by making this connection directly via a separate
wire or PC trace.
Output Clamp
In chopper-stabilized amplifiers, the output clamp pin
reduces overload recovery time. When a connection is
made to the inverting input pin (summing junction), a current
path is created between that point and the output pin, just
before the device output saturates. This prevents uncon-
trolled differential input voltages and charge buildup on
correction-storage capacitors. Output swing is reduced.
Clock
The TC7652 has a 550Hz internal oscillator, which is
divided by two before clocking the input chopper switches.
The 275Hz chopping frequency is available at INT CLK OUT
(pin 12) on 14-pin devices. In normal operation, INT/EXT
(pin 14), which has an internal pull-up, can be left open.
An external clock can also be used. To disable the
internal clock and use an external one, the INT/EXT pin must
be tied to VSS. The external clock signal is then applied to the
EXT CLK IN input (pin 13). An internal divide-by-two pro-
vides a 50% switching duty cycle. The capacitors are only
charged when EXT CLK IN is high, so a 50% to 80% positive
duty cycle is recommended for higher clock frequencies.
The external clock can swing between VDD and VSS, with the
logic threshold about 2.5V below VDD.
The output of the internal oscillator, before the divide-
by-two circuit, is available at EXT CLK IN when INT/EXT is
high or unconnected. This output can serve as the clock
input for a second TC7652 (operating in a master/slave
mode), so that both op amps will clock at the same fre-
quency. This prevents clock intermodulation effects when
two TC7652's are used in a differential amplifier configura-
tion.
TEST CIRCUIT
R2
1 MΩ
R1
1 kΩ
–
+
C
R
C
TC7652
OUTPUT
0.1 µF
0.1 µF
If the TC7652's output saturates, error voltages on the
2 external capacitors will slow overload recovery. This condi-
tion can be avoided if a strobe signal is available. The strobe
signal is applied to EXT CLK IN and the overload signal is
applied to the amplifier while the strobe is LOW. In this case,
neither capacitor will be charged. The low leakage of the
capacitor pins allow long measurements to be made with
negligible errors (typical capacitor drift is 10µV/sec).
APPLICATION NOTES
Component Selection
3
CA and CB (external capacitors) should be in the 0.1µF
to 1µF range. For minimum clock ripple noise, use a 1µF
capacitor in broad bandwidth circuits. For limited bandwidth
applications where clock ripple is filtered out, use a 0.1µF
capacitor for slightly lower offset voltage. High-quality film-
4 type capacitors (polyester or polypropylene) are recom-
mended, although a lower grade (ceramic) may work in
some applications. For quickest settling after initial turn-on,
use low dielectric absorption capacitors (e.g., polypropy-
lene). With ceramic capacitors, settling to 1µV takes
several seconds.
Static Protection
5 Although input diodes static-protect all device pins,
avoid strong electrostatic fields and discharges that can
cause degraded diode junction characteristics and produce
increased input-leakage currents.
Latch-Up
Junction-isolated CMOS circuits have a 4-layer (p-n-
p-n) structure similar to an SCR. Sometimes this junction
6 can be triggered into a low-impedance state and produce
excessive supply current. Therefore, avoid applying voltage
greater than 0.3V beyond the supply rails to any pin. Estab-
lish the amplifier supplies at the same time or before any
input signals are applied. If this is not possible, drive circuits
must limit input current flow to under 1mA to avoid latch-up,
even under fault conditions.
7 Output Stage/Load Driving
The output circuit is high impedance (about 18kΩ). With
lesser loads, the chopper amplifier behaves somewhat like
a transconductance amplifier with an open-loop gain propor-
tional to load resistance. (For example, the open-loop gain
is 17dB lower with a 1kΩ load than with a 10kΩ load.) If the
amp is used only for DC, the DC gain is typically greater than
120dB (even with a 1kΩ load), and this lower gain is
8 inconsequential. For wideband, the best frequency response
occurs with a load resistor of at least 10kΩ. This produces
TELCOM SEMICONDUCTOR, INC.
3-283
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