An300 – Cirrus Logic AN300 REV1 User Manual
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AN300REV1
AN300
amplifier. The chopper amplifier is really a modulation system in which the DC and very low-frequency portion of the
input signal is alternately turned on and off. This results in the signal being modulated at the carrier (chopping) fre-
quency. An oscillator in the chopper amplifier toggles switches S1 and S2 on and off simultaneously. When switch
S1 is conducting, the input to the chopper amplifier is ground. When switch S1 is off, the input signal is allowed to
pass into Amplifier #2. The effect of chopping is to turn the DC input signal into a square wave (AC) signal whose
amplitude changes between ground and the amplitude of the input signal. This signal is then amplified by the AC
amplifier. Once the signal is amplified as an AC signal, it is restored to ground reference by the output chopping
switch. This resulting signal, which is a ground-referenced square wave, is then filtered with a low-pass filter com-
posed of resistors R5 and R6 in conjunction with capacitor C5. The corner frequency of this filter is very low – typi-
cally only fractions of a Hertz. The time constant must be very long to maintain the voltage on the filter capacitor
over the half cycle when the chopped signal is not presented to the output filter.
Figure 2
illustrates the DC amplifier portion of the Zeltex device. This amplifier was constructed with a matched bi-
polar transistor pair as the front end. The open loop gain of the DC amplifier is about 94 dB.
Figure 3
illustrates the actual chopper amplifier portion of the Zeltex amplifier. The amplifier AC gain is about 3000,
but because the signal is on only 50% of the time, the effective gain is only half as much. Therefore the AC amplifier
gain is about 1500, or about 63 dB. The chopping oscillator operates at about 200 Hz. Note that the chopper output
is filtered by a low-pass filter with an extremely low corner frequency.
Figure 2. Amplifier 1: DC Amplifier Portion of the Zeltex Module
-IN
0.005uF
OUT
1N459
51k
2N2914
1
7
3
5
2
6
2N4248
10M
51k
E304
2N4248
100k
10pF
2.4k
680
500k
56k
C1
0.1uF
R1
200k
From AC
Amplifier
+15V
-15V