Application of AD855X series in Weak Signal Detection

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Fan lei, National Defense Science and Technology University, Xie Yuanping

For a large number of weak quantities to be detected, such as low light, weak magnetic, micro-temperature, micro-current, micro-vibration, and small displacement, various sensors are usually used for non-electric conversion, convert the detection object to power (current or voltage ). Because the measured object itself is weak, and the sensitivity of various sensors is limited, the obtained power is still a small signal, which cannot be directly used for sampling. In the circuit design of signal detection, signals usually need to go through first-level or multi-level pre-amplification. At this time, the noise contained in the signal includes two aspects: one is from the external side of the circuit detection system, such as mains interference, temperature fluctuations, and cosmic rays, the method to overcome such noise is to adopt strict electromagnetic shielding; the second is the noise caused by the Front amplification, which cannot be overcome by blocking, only the front-amplifier with superior performance and low-noise Design circuit can be used to minimize the noise impact. The amplifier with superior performance is the key. The AD855X series is a low-noise, zero-drift, single-power amplifier manufactured by AD. Its superior performance makes it the first choice for many signal detection circuits.

1. Problems with General Amplifiers

Figure 1 shows a general amplification circuit. In the figure, ES is the signal source, E1/F is the low-frequency noise of semiconductors, vos a is the inherent offset of the amplifier, Vout is the output signal, and a is the magnification. The output signal can be calculated as follows:

As shown in formula (1), low-frequency noise and inherent bias of the amplifier are also amplified while amplifying the signal, which greatly reduces the signal-to-noise ratio, especially when the signal to be detected is a very small weak signal, the signal after pre-amplification cannot be used at all.

The traditional solution 2 shows that A and B are added to the circuit. The switch is generally controlled by a single-chip microcomputer and other logic circuits. When a is off and B is on, the output is type (1). When a is on and B is off, the output is:

The enlarged signal after the offset is adjusted:

Type, T is the switch Conversion Cycle. This method can effectively suppress low frequency and bias noise. However, because external logic control circuits are required, this increases the components in the circuit and difficulties in control, in addition, a high frequency of control is required for good suppression, which makes this method not feasible in many cases. The AD855X series amplifiers solve these problems and make them the first choice for Low-Frequency Weak Signal pre-amplifier detection.

Performance Characteristics of 2ad855x Series

The AD855X series consists of ad8551, ad8552, and ad8554, which are respectively 1, 2, and 4-channel amplifiers. The AD855X series is a high-precision railtorail Amplifier in the CMO s structure. Only 5 V or 2.7 V single power supply. The voltage offset smaller than 1μv enables it to be set to a large gain without causing a large output voltage error. Very small voltage and temperature offset (5nv/℃) to make it work in-40 ~ + 125 ℃ only has a small voltage offset.

(1) Structure of the AD855X Series

The AD855X series amplifiers are composed of two internal amplifiers, one primary amplifier and one secondary amplifier. The primary amplifier scales up the signal and the secondary amplifier is used to adjust the offset voltage of the primary amplifier, its structure 3 is shown in.

(2) AD855X series amplification principle

In Figure 3, Vin + and Vin-are differential input signals, Vosa and v osb are inherent offsets of a and B amplifiers, and AA and AB are respectively, the gain coefficient of B amplifier. BA and BB are the offset gain coefficient of A and B amplifiers respectively. The status of M and N switches determines the operating stages of the amplifier: offset adjustment stage and amplification stage.

M off, N on, the amplifier is in the adjustment offset phase, as shown in Figure 3:

M-on, N-off, amplifier in the amplification output phase, from Figure 3:

Because ts is much less than the Vosa and vosb change time, it can be considered that v osa and vosb remain unchanged during ts time, and the following can be obtained:

The AD855X series adopts the optimal design, that is, AA = AB, BA = BB> 1. Therefore, the above formula can be reduced:

As can be seen from formula (11), the AD855X series amplifier has a signal magnification of aaab, while the inherent offset of the amplifier (Vosa + vosb) is only AA. It can be seen that the use of a d855x series amplifier will only bring a very small inherent offset of the amplifier.

(3) low-frequency noise characteristics of the AD855X Series

Low-frequency noise is also called 1/f noise because its power spectral density changes according to 1/f. This noise is very common, not only in electronic tubes, semiconductor tubes, diodes and resistors, but also in Thermistor, carbon pickups and optical sources. In semiconductor devices, 1/f noise is mainly caused by the surface properties of semiconductor materials. The carrier excitation and combination in the surface energy state, and the surface state density are the main factors affecting 1/f noise. Generally, low-frequency amplifiers have a great influence on low-frequency noise. The AD855X amplifier also has the advantage of low-frequency noise suppression. Because the AD855X series amplifier has the automatic offset adjustment function, the low-frequency noise produced by the semiconductor tube will be restrained like the inherent offset. This allows the AD855X series amplifier to be used for low-frequency or even DC signals without causing large low-frequency noise.

3ad8554

Accelerometer and pressure sensor are commonly used sensing systems. Most stress sensing systems have low-frequency and weak signals, but almost all of them have high requirements on the linear degree of scale. This makes the offset and drift of the front amplifier very important.

Figure 4 shows the front-discharge circuit of the bridge stress sensing system. In most cases, the bridge outputs a signal of dozens of millivolts. Therefore, a small inherent offset of the amplifier will greatly affect the signal detection, the ad8554 Automatic offset adjustment meets this requirement. In addition, the ad8554 is a 4-channel amplifier that makes the entire front-end system have fewer devices and the circuit is simple.

Another advantage of using ad8554 in the bridge stress sensing system is that a small input current allows the use of a bridge with a large resistance during the design process to improve the detection accuracy, A small input current prevents the bridge from the measurement error caused by temperature changes due to heat. Low Power Consumption, suitable for low power consumption and portable systems.

4 Conclusion

Simulating the quality of front-end in the weak signal detection is the key to the entire detection system. Ad produces AD855X series Amplifiers with its low noise, zero drift, single power supply, the advantage of large input resistance can provide a good choice for Low-Frequency Weak Signal Detection. Through the analysis of its working principle and a specific example, it illustrates its superior performance.