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CS3001/2/11/12 & CS3003/4/13/14 Chopper-stabilized

Operational Amplifiers

Jerome E. Johnston

1. INTRODUCTION

Cirrus Logic offers a variety of low-voltage CMOS chopper-stabilized amplifiers.

The chopper-stabilized amplifiers designed at Cirrus Logic are unique. These amplifiers offer performance benefits

that combine the best features of bipolar amplifiers with the best features of chopper amplifiers. The intent of this

application note is to understand Cirrus Logic's unique technology and to see how it can be applied in various mea-

surement applications. But before the applications are discussed, the following provides a brief overview of the con-

cepts involved in a chopper-stabilized amplifier.

2. CHOPPER AMPLIFIER AND CHOPPER-STABILIZED AMPLIFIER BASICS

Not everyone is familiar with chopper amplifiers and chopper-stabilized amplifiers. A look back at some history can

help us understand how the chopper-stabilized amplifier operates.

Figure 1 illustrates the block diagram of a chopper-stabilized amplifier. A chopper-stabilized amplifier is a DC am-

plifier whose offset is stabilized by a chopping amplifier. The basic amplifier diagram in Figure 1 is called the Gold-

berg configuration, named after E. A. Goldberg, an engineer who designed and patented electron tube-based,

chopper-stabilized amplifiers for RCA (Radio Corporation of America) in the 1940s and 1950s.

Figure 1. Basic Chopper-stabilized Amplifier Block Diagram

The Goldberg configuration was later used in a transistorized chopper-stabilized amplifier designed and sold

by Zeltex Corporation in the 1970s for about $125 (US). The Zeltex chopper-stabilized amplifier follows the basic

block diagram of Figure 1. The input signal (e

) at the inverting input of the amplifier travels through two different

signal paths. (The Goldberg configuration amplifier could only be used as an inverting amplifier.) The first path is

into Amplifier #1 of Figure 1. Components R1-C1 act as a high-pass filter that prevents the DC portion of the signal

from passing directly into Amplifier #1. The second path is through a low-pass filter composed of R2 and C2. The

R2-C2 filter limits the bandwidth of the signal to be chopped by the chopper amplifier. Amplifier #2 is the chopper

AMP2

AMP1

R3R2

S2S1

Oscillator

AC Amplifier (Chopper)

DC Amplifier

out

Low-pass

Filter

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AN300REV1

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Inhaltsverzeichnis

Seite 1 - Operational Amplifiers

Seite 2

10 AN300REV1AN300Figures 13 and 14 illustrate the noise performance of the CS3001/2 amplifiers. Figure 13 illustrates the extremelow-frequency noise p

Seite 3

AN300REV1 11AN300Figure 15 illustrates the offset stability of the CS3001/2 over time. This plot was based upon band limiting theamplifier to 3.2 Hz a

Seite 4

12 AN300REV1AN300Device Amps GBWSlew Rate(V/ S)AVOL(dB)VoltageNoise0.1 to 10 HzNoiseIbiasVOS(V)Drift(V/°C)SupplyRangeIq/Ch(mA)R-to-RInputR-to-ROutputC

Seite 5 - MOS Transisitor Noise

AN300REV1 13AN3004. APPLICATION CIRCUITS4.1 Thermopile AmplifiersFigures 16 illustrates a basic diagram of a thermopile. The thermopile is a collectio

Seite 6 - ≈ 100 dB

14 AN300REV1AN300Figure 18 illustrates a thermopile amplifier set for a gain of 640 V/V. The Dexter Research 1M thermopile is a thinfilm device that e

Seite 7

AN300REV1 15AN300The circuit illustrates the CS3001 with a gain of negative 408X. Note that the negative output from the bridge is con-nected to the i

Seite 8

16 AN300REV1AN300Contacting Cirrus Logic SupportFor all product questions and inquiries, contact a Cirrus Logic Sales Representative. To find the one

Seite 9 - AN300REV1 9

2 AN300REV1AN300amplifier. The chopper amplifier is really a modulation system in which the DC and very low-frequency portion of theinput signal is al

Seite 10

AN300REV1 3AN300Figure 3. Amplifier 2: Chopper Amplifier Portion of the Zeltex ModuleThe gain transfer functions of the two amplifiers are multiplied

Seite 11

4 AN300REV1AN300Figure 4. Chopper-stabilized Amplifier Using Electron Tubes and Mechanical Vibrating Switches3. MODERN CHOPPER-STABILIZED AMPLIFIERSM

Seite 12 - 12 AN300REV1

AN300REV1 5AN300As mentioned previously, bipolar transistors have long dominated the world of operational amplifiers. Let's look atsome of the di

Seite 13 - 4. APPLICATION CIRCUITS

6 AN300REV1AN300operating current. But there is another approach to achieve low noise in a CMOS amplifier. That is to modulate(chop) the input signal

Seite 14 - 4.2 Load Cell Amplifiers

AN300REV1 7AN300inside the Cirrus Logic CS3001/2/11/12 amplifiers. Note that although not shown in the diagram, the signal ischopped as a differential

Seite 15 - CS5510/12

8 AN300REV1AN300Figure 11. Open-loop Gain and Phase vs. Frequency, CS3001/02/11/12 (Expanded)In applications that require a small signal (< 10 mV)

Seite 16

AN300REV1 9AN300Figure 12. Open Loop Gain ComparisonIf an operational amplifier with only 120 dB of open loop gain is configured for a forward gain o