DN42 - Chopper vs Bipolar Op Amps—An Unbiased Comparison

Chopper vs Bipolar Op Amps—An Unbiased Comparison
Design Note 42
George Erdi
Table 1 lists the parameters of importance. In all input
parameters (except noise) the advantage unquestionably goes to the choppers. 5μV maximum offset voltage, 0.5μV/°C maximum drift are commonly found
Table 1. Chopper Stabilized vs Precision Bipolar Op Amps
ADVANTAGE
PARAMETER
Offset Voltage
Offset Drift
All Other DC Specs
CHOPPER BIPOLAR COMMENTS
✓
✓
✓
No Contest
Wideband, 20Hz to
1MHz
✓
See Details in Text
Noise
✓
See Details in Text
✓
Rail to Rail Swing
2mA Limit on
Choppers
Output: Light Load
Heavy Load
Single Supply
Application
✓
✓
Inherent to
Choppers Needs
Special Design
Bipolars
±15V Supply Voltage
✓
Except LTC1150
Prejudice/Tradition
✓
Still a Chopper
Problem
Cost
08/90/86_conv
✓
Unless DC
Performance
Needed
guaranteed parameters on all Linear Technology
choppers. Changes with time and temperature cycling
are near zero. These parameters cannot be measured
accurately, but can be guaranteed by design; assuming that the auto-zeroing chopper loop, which can be
tested independently, is working properly. The best,
tightly specified bipolar op amps can only approach
this performance, at the cost of great testing and yield
expense.
In wideband applications bipolars get the nod. This
may seem inconsistent, since typical chopper slew
rate is 4V/μs, bandwidth is 2.5MHz –faster than most
precision op amps. But choppers have clock frequency
spikes, chopping frequency spikes, aliasing errors,
millisecond overload recovery, and high wideband
noise. All these factors limit the choppers’ usefulness
as wideband amplifiers.
The noise performance of bipolars is acknowledged to
be superior. As shown in Figure 1 from 10Hz to 1kHz
bipolar noise is nine times better. This comparison is
for the industry standard LT®1001 and OP-07. Bipolar
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
120
VOLTAGE NOISE DENSITY (nV/√Hz)
Over the last few years dozens of new CMOS chopper
stabilized and precision bipolar op amps have been
introduced. Despite the fact that these two groups
compete for the same market, a valid scientific comparison of the merits of choppers and precision bipolar
is unavailable. The probable explanation is that most
analog IC companies have introduced products in one
group or the other but not both. Therefore, articles
and news releases have extolled the benefits of one,
while knocking the other. Linear Technology is the only
company with offerings in both groups with no vested
interest in promoting one versus the other. Hence, an
attempt will be made for an unbiased comparison.
100
CHOPPER
80
60
WIDEBAND: BIPOLAR 9x BETTER
≤10Hz: BIPOLAR 5x BETTER
≤1Hz: BIPOLAR 2x BETTER
≤0.25Hz: CHOPPER BETTER
40
20
BIPOLAR
0
0.01 0.1
1
10 100
1k
FREQUENCY (Hz)
10k
100k
DN042 F01
Figure 1. Bipolar vs Chopper Noise Comparison
Table 2. Chopper Stabilized Op Amps
MAX VOS
(25°C)
MAX
TCVOS
TYPICAL 0.1Hz
TO 10Hz NOISE
EXTERNAL
CAPS
REQUIRED
MAXIMUM
SUPPLY
VOLTAGE
Single, Micropower
10μV
0.10μV/°C
3.0μVp-p
No
±9V
LTC1050
Single, Low Power
5μV
0.05μV/°C
1.6μVp-p
No
±9V
LTC1051
Dual, Low Power
5μV
0.05μV/°C
1.5μVp-p
No
±9V
LTC1052
Single, 7652 Upgrade
5μV
0.05μV/°C
1.5μVp-p
Yes
±9V
LTC1053
Quad, Low Power
5μV
0.05μV/°C
1.5μVp-p
No
±9V
LTC1150
Single, ±15V Operation
5μV
0.05μV/°C
1.8μVp-p
No
±18V
PART
NUMBER
DESCRIPTION
LTC1049
Table 3. Precision Bipolar Op Amps
DESCRIPTION
SINGLE
DUAL
QUAD
Low Cost, Optimum
Performance
LT1001
LT1012
LT1097
LT1013
LT1078
LT1014
LT1079
Low Noise, Wideband
LT1007
LT1028
LT1037
Low Noise, Audio
LT1115
Single Supply, Low Power
LT1006
LT1013
LT1014
Single Supply, Micropower
LT1077
LT1078
LT1178
LT1079
LT1179
designs optimized for low noise, such as the LT1007,
LT1028, LT1037, or LT1115, have 36 to 100 times lower
noise than choppers. But choppers do not have 1/f noise,
i.e. as frequency decreases bipolar noise increases,
while chopper noise stays flat. If the bandwidth is limited chopper noise gets comparatively better. If signal
bandwidth is cut-off at 0.25Hz—a rather restrictive
requirement—chopper noise is actually lower.
Chopper stabilized amplifiers are also limited to ±9V
maximum supplies, excluding them from the mainstream ±15V analog applications. The new LTC ®1150
is the exception. The LTC1150 represents a major
breakthrough; it plugs into standard ±15V sockets, yet
guarantees the expected 5μV offset and 0.05μV/°C drift.
A non-scientific, yet real, parameter of comparison
is prejudice/tradition. Early CMOS circuits have
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established a reputation of being damaged easily by
electrostatic discharge, and latching up under normal
operating conditions. Most of the problems were solved
years ago, yet the negative impression lingers. Many
system designers will not try, and therefore will not
use, CMOS choppers.
The cost of precision bipolar op amps is lower than
choppers. For example, the 1000 piece price of the
LT1097CN8 (50μV max offset voltage, 1μV/°C max drift)
is $0.97 versus the LTC1050CN8’s $2.10. This, however,
is somewhat of an apples to oranges comparison, because the LTC1050CN8’s offset and drift performance
cannot be obtained at any price on a bipolar op amp.
Table 2 summarizes Linear Technology’s chopperstabilized op amp offerings. Table 3 lists the currently
available precision bipolar operational amplifiers.
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