tc7650 Microchip Technology Inc., tc7650 Datasheet - Page 5

• Manufacturer Part Number: tc7650
• Description: Chopper Stabilized Operational Amplifier
• Manufacturer: Microchip Technology Inc.

tion region, where the main amplifier takes over from
the null amplifier. The clock frequency sets the transi-
tion region.
3.3
Previous chopper stabilized amplifiers have suffered
from intermodulation effects between the chopper fre-
quency and input signals. These arise because the
finite AC gain of the amplifier results in a small AC sig-
nal at the input. This is seen by the zeroing circuit as an
error signal, which is chopped and fed back, thus inject-
FIGURE 3-1:
FIGURE 3-2:
3.4
The offset voltage correction capacitors are connected
to C
made to V
capacitor return (C
The common connection should be made through a
separate PC trace or wire to avoid voltage drops. The
capacitors outside foil, if possible, should be connected
to C
©
2002 Microchip Technology Inc.
Analog Input
RETN
A
5
4
and C
14-PIN PACKAGE
2
-
+
Intermodulation
Nulling Capacitor Connection
TC7650
C
or V
V
DD
A
SS
V +
V -
11
B
C
SS
8
V
. The common capacitor connection is
(Pin 4) on the 8-pin packages and to
B
SS
.
7
1
RETN
10
TC7650 CONTAINS A NULLING AND MAIN AMPLIFIER. OFFSET CORRECTION
VOLTAGES ARE STORED ON TWO EXTERNAL CAPACITORS
NULLING CAPACITOR
CONNECTION
, Pin 8) on the 14-pin packages.
A
3
2
B
8-PIN PACKAGE
1
TC7650
+
-
V
8
DD
C
C
7
B
A
4
6
Gain = A
+
-
Null
V
Null
Amplifier
SS
TC7650
N
, Offset = V
ing sum and difference frequencies, and causing dis-
turbances to the gain and phase versus frequency
characteristics near the chopping frequency. These
effects are substantially reduced in the TC7650 by
feeding the nulling circuit with a dynamic current corre-
sponding to the compensation capacitor current in such
a way as to cancel that portion of the input signal due
to a finite AC gain. The intermodulation and gain/phase
disturbances are held to very low values, and can gen-
erally be ignored.
3.5
The internal oscillator is set for a 200Hz nominal chop-
ping frequency on both the 8- and 14-pin DIPs. With the
14-pin DIP TC7650, the 200 Hz internal chopping fre-
quency is available at the internal clock output (Pin 12).
A 400Hz nominal signal will be present at the external
clock input pin (Pin 13) with INT/EXT high or open. This
is the internal clock signal before a divide-by-two oper-
ation.
The 14-pin DIP device can be driven by an external
clock. The INT/EXT input (Pin 14) has an internal pull-
up and may be left open for internal clock operation. If
an external clock is used, INT/EXT must be tied to V
(Pin 7) to disable the internal clock. The external clock
signal is applied to the external clock input (Pin 13).
The external clock amplitude should swing between
V
between V
At low frequencies the external clock duty cycle is not
critical, since an internal divide-by-two gives the
desired 50% switching duty cycle. The offset storage
correction capacitors are charged only when the exter-
nal clock input is high. A 50% to 80% external clock
DD
and ground for power supplies up to ±6V and
B
Clock Operation
OSN
A
+
and V
+
-6V for higher supply voltages.
+
Null
-
.
Gain = A
Main
Amplifier
TC7650
M
DS21463B-page 5
C
C
B
A
V
OUT
SS