MT-055
TUTORIAL Chopper Stabilized (Auto-Zero) Precision Op Amps
CHOPPER AMPLIFIERS
For the lowest offt and drift performance, chopper-stabilized (auto-zero) amplifiers may be the only solution. The best bipolar amplifiers offer offt voltages of 25 µV and 0.1 µV/ºC drift. Offt voltages less than 5 µV with practically no measurable offt drift are obtainable with choppers, albeit with some penalties.
A basic chopper amplifier circuit is shown in Figure 1 below. When the switches are in the "Z" (auto-zero) position, capacitors C2 and C3 are charged to the amplifier input and output offt voltage, respectively. When the switches are in the "S" (sample) position, V IN is connected to V OUT through the path comprid of R1, R2, C2, the amplifier, C3, and R3. The chopping frequency is usually between a few hundred Hz and veral kHz, and it should be noted that becau this is a sampling system, the input frequency must be much less than one-half the chopping frequency in order to prevent errors due to aliasing. The R1-C1 combination rves as an antialiasing filter. It is also assumed that after a steady state condition is reached, there is only a minimal amount of charge transf
erred during the switching cycles. The output capacitor, C4, and the load, R L, must be chon such that there is minimal V OUT droop during the auto-zero cycle.
Figure 1: Classic Chopper Amplifier
AUTO-ZERO CHOPPER STABILIZED OP AMP
The basic chopper amplifier of Fig. 1 can pass only very low frequencies becau of the input filtering required to prevent aliasing. In contrast to this, the chopper-stabilized architecture shown in Figure 2 is most often ud in chopper amplifier implementations.
–IN
+IN
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Figure 2: Modern Auto-Zero (Chopper-Stabilized) Op Amp
In this circuit, A1 is the main amplifier, and A2 is the nulling amplifier. In the sample mode (switches i
n "S" position), the nulling amplifier, A2, monitors the input offt voltage of A1 and drives its output to zero by applying a suitable correcting voltage at A1's null pin. Note, however, that A2 also has an input offt voltage, so it must correct its own error before attempting to null A1's offt. This is achieved in the auto-zero mode (switches in "Z" position) by momentarily disconnecting A2 from A1, shorting its inputs together, and coupling its output to its own null pin. During the auto-zero mode, the correction voltage for A1 is momentarily held by C1. Similarly, C2 holds the correction voltage for A2 during the sample mode. In modern IC chopper-stabilized op amps, the storage capacitors C1 and C2 are on-chip.
Note in this architecture that the input signal is always connected to the output, through A1. The bandwidth of A1 thus determines the overall signal bandwidth, and the input signal is not limited to less than one-half the chopping frequency as in the ca of the traditional chopper amplifier architecture. However, the switching action does produce small transients at the chopping frequency, that can mix with the input signal frequency and produce intermodulation distortion.
A patented spread-spectrum technique is ud in the AD8571/AD8572/AD8574 ries of single-supply chopper-stabilized op amps, to virtually eliminate intermodulation effects. The devices u a pudorandom chopping frequency swept between 2 kHz and 4 kHz. Figure 3 compares the inter
modulation distortion of a traditional chopper stabilized op amp.
The (AD8551/AD8552/AD8554, left) us a fixed 4 kHz chopping frequency, and the AD8571/AD8572/AD8574 (right) us the pudorandom chopping frequency.
AD8551/52/54
FIXED CHOPPING FREQUENCY:
4kHz
AD8571/72/74
PSEUDORANDOM CHOPPING FREQ:
2kHz -4kHz
显示器花屏INPUT SIGNAL = 1mV RMS, 200Hz
OUTPUT SIGNAL: 1V RMS, 200Hz
工业安全GAIN = 60dB
V S= +5V
G = 60dB黄连阿胶鸡子黄汤
V S= +5V
G = 60dB
Figure 3: Intermodulation Product: Fixed Versus
Pudorandom Chopping Frequency
A comparison between fixed and pudorandom chopping on the voltage noi is shown in Figure 4 below. Notice for the fixed chopping frequency, there are distinct peaks in the noi spectrum at the odd harmonics of 4 kHz, whereas with pudorandom chopping, the spectrum is much more uniform, although the average noi level is higher.
山村变了样
AD8551/52/54
荒诞无稽
FIXED CHOPPING FREQUENCY:
4kHz
AD8571/72/74
PSEUDORANDOM CHOPPING FREQUENCY
2kHz -4kHz
V S= +5V
R S= 0Ω
V S= +5V
R S= 0Ω
Figure 4: Voltage Noi Spectral Density Comparison: Fixed Versus
Pudorandom Chopping Frequency
个人政治表现怎么写Another method for reducing the intermodulation effects the switching action of auto-zero amplifiers is through a patented combination of auto-zeroing and chopping as ud in the AD8628/AD8629/AD8630 family. This unique topology allows the amplifiers to maintain their low offt voltage over a wide temperature range and over their operating lifetime.
The AD8628/AD8629/AD8630 also optimize the noi and bandwidth over previous generations of auto-zero amplifiers, offering the lowest voltage noi of any auto-zero amplifier by more than 50%. Other designs u either auto-zeroing or chopping to add precision to the specifications of an amplifier. Auto-zeroing results in low noi energy at the auto-zeroing frequency, at the expen of higher low frequency noi due to aliasing of wideband noi into the auto-zeroed frequency band. Chopping results in lower low frequency noi at the expen of larger noi energy at the chopping frequency.
The AD8628/AD8629/AD8630 family us both auto-zeroing and chopping in a patented "ping-pong" arrangement to obtain lower low frequency noi together with lower energy at the chopping and auto-zeroing frequencies, maximizing the signal-to-noi ratio for the majority of applications without
the need for additional filtering. The relatively high clock frequency of 15 kHz simplifies filter requirements for a wide, uful, noi-free bandwidth. The noi spectral density of the family is shown in Figure 5.
DC TO 2.5kHz DC TO 25kHz
光与爱
Figure 5: Voltage Noi Spectral Density of AD8628/AD8629/AD8630 Family
of Precision Zero-Drift, Auto-Zero Op Amps
The AD8628 is among the few auto-zero amplifiers offered in the 5-lead TSOT package. This provides a significant improvement over the ac parameters of the previous auto-zero amplifiers. The AD8628/AD8629/AD8630 have low noi over a relatively wide bandwidth (0 Hz to 10 kHz) and can be ud where the highest dc precision is required. In systems with signal bandwidths of from 5 kHz to 10 kHz, the AD8628/AD8629/AD8630 provide true 16-bit accuracy, making them the best choice for very high resolution systems.
Key features of the AD8628/AD8629/AD8630 family are shown in Figure 6.
Single Supply: +2.7V to +5V
1µV Typical Input Offt Voltage
0.002µV/°C Typical Input Offt Voltage Drift
130dB Typical CMR, PSR
0.85mA Typical Supply Current/Amplifier
10µs Overload Recovery Time
22nV/√Hz Input Voltage Noi @ 1kHz
Patented Auto-Zero and Chopper-Stabilized Technique @ 15kHz Switching Frequency
2.5MHz Gain-Bandwidth Product
AD8628 (Single) in TSOT and SOT-23 Packages
AD8629 (Dual), AD8630 (Quad)
Figure 6: Key Features of the AD8628/29/30 Family of
Precision Auto-Zero Op Amps
It should be noted that extreme care must be taken when applying all of the chopper stabilized devices. This is becau in order to fully realize the full offt and drift performance inherent to the parts, parasitic thermocouple effects in external circuitry must be avoided.
NOISE CONSIDERATIONS FOR CHOPPER-STABILIZED OP AMPS
It is interesting to consider the effects of a chopper amplifier on low frequency 1/f noi. If the chopping frequency is considerably higher than the 1/f corner frequency of the input noi, the chopper-stabilized amplifier continuously nulls out the 1/f noi on a sample-by-sample basis. Theoretically, a chopper op amp therefore has no 1/f noi. However, the chopping action produces wideband noi which is generally much wor than that of a precision bipolar op amp. Figure 7 below shows the noi of a precision bipolar amplifier (OP177) versus that of the AD8628/AD8629/AD8630 chopper-stabilized op amp. The peak-to-peak noi in various bandwidths is calculated for each in the table below the graphs.
