无需其它器件的低噪声自调零放大器

　Analog Devices AD8553仪表放大器的独特架构降低并联设备噪声。

　　Analog Devices AD8553自调零仪表放大器具有独特架构，其两个增益设置电阻没有公共结点（参考文献1）。IC的前端是一个精密电压电流转换器，其中一个增益设置电阻R₁设置跨导的大小。IC的末端是一个精密电流电压转换器，它的反馈电阻R₂的值，根据G=2(R₂/R₁)共同决定全部电压增益。可以发现，两个增益设置电阻是独立的，输入端的电压控制电流源，用减少放大器数量的方法，满足严格的低噪声要求。

　　多使用放大器减小噪声分两步。首先，假设放大器的随机噪声源相互独立。进一步，假设噪声服从高斯分布。当平均经典电压放大器的输出时，通过使用N个放大器和三倍电阻减少噪声到1/ （参考文献2）。AD8553内部架构对几乎无限个并联IC工作时，仅允许使用N+1个电阻。通过并联更多IC各自的输入引脚，连接内部电压电流源容易并联工作（图1）。微伏级的输入电压偏置与若干IC的并联输入引脚配合不当是无害的，因为电压电流转换器的输出电阻理论上是无穷大的。

　　并联N次输入端的网络结果是单IC输出电流的N(V_INP–V_INN)/(2R₁)或N倍。可以仅使用N个IC电流电压端的一个。端反馈电阻为R₂/N，在此，R₂为单IC的期望电压增益A_V值。由于放大器噪声的主要来源为输入端，假设N个并联电压电流转换器输出电流的随机器件标准差为σ_NI=σ_I× ，σ_I为电压电流转换器输出电流的随机器件标准差。这些结果与参考文献2中的不同，文献中作者通过多电压平均的方法实现减小噪声。另一方面，图1电压电流转换器的共模输出中电流的决定成分为单IC的N倍。下面的公式计算RSNR（相对信噪比），定义超过输出噪声标准差的输出电流：RSNR_N=(N×I)/(σI×)=×RSNR₁。实际上，意味着电路噪声减少到单IC的1/ 。[!--empirenews.page--]

       英文原文：

　　Autozeroed amplifier with halved noise needs few components

　　The unique architecture of the Analog Devices AD8553 instrumentation amplifier permits paralleling devices to lower noise.

　　Marián Štofka, Slovak University of Technology, Bratislava, Slovakia; Edited by Charles H Small and Fran Granville -- EDN, 10/25/2007

　　The Analog Devices AD8553 autozeroed instrumentation amplifier has a unique architecture in that its two gain-setting resistors have no common junction (Reference 1). The first stage of the IC is a precise voltage-to-current converter, in which the first gain-setting resistor, R1, sets the magnitude of the transconductance. The end stage of the IC is a precise current-to-voltage converter, in which the value of its feedback resistor, R2, co-determines the overall voltage gain as G="2"(R2/R1). You can exploit the fact that the two gain-setting resistors are separate and that the input stage is a voltage-controlled current source to lower the component count in amplifiers with extreme noise-reduction demands.

　　You can use more amplifiers to reduce noise in two ways. First, assume that the sources of random noise in the amplifiers are mutually independent. Further, assume that the noise obeys a gaussian distribution. When averaging the outputs of classic voltage amplifiers, you can reduce the noise to a fraction of 1/ by using N amplifiers and three times as many resistors (Reference 2). The internal structure of the AD8553 allows you to use just N+1 resistors for an almost-unlimited number of ICs operating in parallel. By paralleling the respective input pins of more ICs, the connected internal voltage-to-current sources easily operate in parallel (Figure 1). The microvolt-range input-voltage-offset mismatch at paralleled input pins of several ICs is harmless here because the output resistances of the voltage-to-current converters are theoretically infinite.

　　The net result of paralleling N input stages is that they output current of N(VINP–VINN)/(2R1), or N times that of a single IC. You use only one of the current-to-voltage stages of the N ICs. That stage’s feedback resistor has the value of R2/N, where R2 is the value for a desired voltage gain of AV in a single IC. Because the primary source of noise in an amplifying IC is its input stage, you can assume that the standard deviation of the random component of output current of the paralleled-N voltage-to-current converters is σNI=σI×, where σI is the standard deviation of the random component of output current of a voltage-to-current converter. These results differ from those in Reference 2, in which the authors perform noise reduction by averaging multiple voltages. On the other hand, the deterministic part of current at the common output of the voltage-to-current converters in Figure 1 has the value of N times that of the single IC. The following equation calculates the RSNR (relative signal-to-noise ratio),
which you define as the output current over the standard deviation of output noise: RSNRN=(N×I)/(σI×)=×RSNR1. It means that, in effect, the noise of the circuit has decreased to a fraction of 1/ compared with that of a single IC.

　　References

　　“AD8553 1.8V to 5V Auto-Zero, In-Amp with Shutdown,” Analog Devices, 2005.

　　Štofka, Marián, “Paralleling decreases autozero-amplifier noise by a factor of two,” EDN, June 7, 2007, pg 94.