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 Data Sheet Download www.linear.com Linear Technology Corporation 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. For applications help, call (408) 432-1900 dn42f_conv IM/GP 1290 • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 1990