Auto-zero amplifier

An auto-zero amplifier is an electronic circuit for amplifying very small DC voltages in measurement technology . Usually these are special operational amplifiers , but there are also corresponding instrument amplifiers . The essential property is a very low and stable offset voltage .

Historical

Since the early days of electronics, attempts have been made to get the problem of the very annoying offset voltage of DC voltage amplifiers under control. The first success achieved EA Goldberg, who had patented in 1948 a chopper stabilized amplifier tube while on the already patented in 1918 by JW Milnor amplifier chopper fell back. In his circuit, the signal path is divided into two paths:

• A capacitor-coupled branch only amplifies the AC voltage component with the lowest possible cutoff frequency.
• The even lower frequencies and the DC voltage component are amplified with a parallel chopper amplifier with zero offset voltage and freed from interference peaks with a subsequent low-pass filter .
• The output voltage is the sum of the two individual signals.

If the cut-off frequencies and individual amplifications are carefully coordinated, a continuous frequency range from 0 Hz to a few 100 kHz can be achieved. The great advantage of this method compared to chopper amplifiers is the significantly higher processable maximum frequency. The lack of long-term stability, the use of mechanical chopper and large-volume tubes, however, hindered their use.

Circuit with semiconductors

Basic circuit of a chopper-stabilized amplifier

With semiconductors it is possible to combine the complete circuit of an auto-zero amplifier into an integrated circuit . Instead of the mechanical chopper from the tube era, MOSFETs serve as switches. As a component in the smallest of spaces, similar to a normal operational amplifier, this enables the use of auto-zero amplifiers as a standard component in measurement technology.

Today's auto-zero amplifiers are internally composed of two amplifier chains:

• The signal is amplified in the frequency range from 0 Hz to about 2 MHz with a low-noise operational amplifier A without coupling capacitors , the output signal of which is distorted by zero point drift and offset without further corrective measures . Because these disruptive influences change very slowly (typical time constant: a few minutes), both are corrected with a large time constant via auxiliary inputs.
• In a separate channel, the low-pass filtered input signal is amplified in two stages (chopper amplifier B and operational amplifier C) and dictates the mean output voltage of operational amplifier A via auxiliary inputs. This compensates for its offset voltage almost perfectly.

As an alternative to the chopper amplifier, the offset can also be corrected using a slightly different method: Here, too, an auxiliary amplifier is parallel. Its inputs are short-circuited at regular intervals in order to store the offset of the auxiliary amplifier in a capacitor and thus correct it. In a second phase, the offset of the main amplifier can then be amplified and corrected without offset. This 2nd variant is the auto-zero amplifier in the real sense. Because of their similar properties, the two terms auto-zero amplifier and chopper stabilized amplifier are often used synonymously, despite their somewhat different internal structure.

A comparison of the technical data confirms the improvement:

OP type	Offset voltage	Temperature dependence	Noise (BW = 1 Hz)
LM11 OP with very little bias	± 300 µV	3 µV / K	6 µV pp
LT1012 OP with low bias	0± 35 µV	1.5 µV / K	0.5 µV pp
AD624 instrumentation amplifier	0± 25 µV	0.25 µV / K	0.2 µV pp
LTC1052 Auto-Zero, still with external capacitors	00± 5 µV	0.05 µV / K	0.5 µV pp
CS3001 chopper stabilized , low noise	0± 10 µV	0.05 µV / K	0.04 µV pp
iC-Haus HQ Auto-Zero?	00± 1 µV	0.01 µV / K	0.6 µV pp

Applications

The applications are in the field of measurement technology for the amplification of direct voltages or low-frequency alternating voltages, such as voltage measuring devices with resolution in the microvolt range , strain gauges , thermocouples and Hall sensors .

Web links

• Auto-zero amplifier (English) (64 kB; PDF file)
• Chopper stabilized amplifier vs. Auto Zero (English) (279 kB; PDF file)

Individual evidence

1. ↑ CHOPPER AMPLIFIER AND CHOPPER-STABILIZED AMPLIFIER BASICS ( Memento of the original from October 29, 2013 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 622 kB)  
2. ↑ Alberto Bilotti and Gerardo Monreal: Chopper-Stabilized Amplifiers With a Track-and-Hold Signal Demodulator, IEEE Transactions on Circuits and Systems, I: Fundamental Theory and Applications, Vol. 46, No. April 4, 1999.
3. ↑ Application Considerations and Circuits for a New Chopper-Stabilized Op Amp ( Memento of the original from March 2, 2013 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 625 kB)  
4. ↑ CS3001 / 2/11/12 & CS3003 / 4/13/14 Chopper-stabilized Operational Amplifiers ( Memento of the original from October 29, 2013 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 622 kB)  
5. ↑ Datasheet AD624 (PDF; 397 kB)
6. ↑ Datasheet iC-Haus HQ and HQL (PDF; 397 kB)