AN-1131
APPLICATION NOTE
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Chopping on the AD7190, AD7192, AD7193, AD7194, and AD7195
by Mary McCarthy
Rev. 0 | Page 1 of 4
INTRODUCTION
Chopping is a technique used to minimize offset error, offset
error drift, and other low frequency errors. This application
note describes how chopping is implemented and highlights the
benefits of chopping.
OFFSET ERRORS
Offset voltage errors arise at many points within a signal
processing chain, such as the temperature-dependent
thermocouple voltage that arises when two dissimilar metals
are joined. Within an integrated circuit such as an ADC, there
are numerous sources of internal offset errors, such as offset
due to the instrumentation amplifier, charge injection onto
the sampling capacitor when a sampling switch is closed, or
interference from EMI radiation. These offsets are generally
undesirable and are problematic if they change with temper-
ature since a one-time calibration is not sufficient to remove
offset errors across all temperatures and power
supplies.
CHOPPING
An offset that arises within the various amplifiers of the Σ-Δ
modulator can generally be nulled out, either by local chopping
or auto-zeroing the amplifier. However, other offset errors
cannot be removed by these methods. The solution imple-
mented on these ADCs is to chop the entire analog signal chain
within the ADCs. This removes any offset and low frequency
errors, resulting in extremely low offset errors and offset error
drift. The chopping scheme is shown in Figure 1.
0
1
0
1
0
1
–+
V
OS
A
IN
(+)
A
IN
(–)
V
IN
(+)
V
IN
(–)
Σ-∆ MODULATOR
–1
TO DIGITAL
FILTER
OUTPUT MUX
INPUT MUX
CHOP
10253-001
Figure 1. Chopping
The differential input to the modulator is alternately inverted
(or chopped) at the input multiplexer and an ADC conversion
is performed for each phase of chop. The modulator chopping
is reversed in the output multiplexer before passing to the
digital filter.
If the offset in the Σ−Δ modulator is represented as V
OS
, then
the output when chop is 0 is
(A
IN
(+) − A
IN
(−)) + V
OS
and the output when the chip is 1 is
−[(A
IN
(−) − A
IN
(+)) + V
OS
]
The error voltage, V
OS
, is removed by averaging these two
results in the digital filter giving
(A
IN
(+) − A
IN
(−))
which equals the differential input voltage without any
offset term.
RESULT/CALIBRATION
The effect is to virtually eliminate any offset errors arising
within the ADC, and more importantly, to minimize any offset
drift due to temperature. The drift of these ADCs is specified
as ±5 nV/°C typical. In fact, it is practically unmeasurable. The
entire analog circuitry is chopped from the input multiplexer to
the modulator output. ADC offset calibrations are, therefore,
not required.
The offset error specification of the ADC is ±0.5 μV typical.
This is measured with 0 V applied externally to the part sitting
in a PCB socket and, therefore, mostly comprises thermocouple
errors due to different metals used in the various contacts (PCB
track, solder, lead frame, bond wire, die metallization, and
so on).
AN-1131 Application Note
Rev. 0 | Page 2 of 4
TABLE OF CONTENTS
Introduction ...................................................................................... 1
Offset Errors...................................................................................... 1
Chopping ........................................................................................... 1
Result/Calibration ............................................................................ 1
Understanding the Step Response.................................................. 3
Output Data Rate...............................................................................3
Frequency Response..........................................................................3
RMS Noise..........................................................................................4
Conclusion..........................................................................................4
REVISION HISTORY
10/11—Revision 0: Initial Version