Low-Noise Auto-Zero Amplifier Requires No Additional Components

The unique architecture of the Analog Devices AD8553 instrumentation amplifier reduces noise from paralleled devices.

The Analog Devices AD8553 auto-zero instrumentation amplifier has a unique architecture in which its two gain-setting resistors have no common node. The front end of the IC is a precision voltage-to-current converter, where one gain-setting resistor, R1, sets the magnitude of the transconductance. The end of the IC is a precision current-to-voltage converter, whose feedback resistor, R2, determines the overall voltage gain according to G = 2(R2/R1). It can be seen that the two gain-setting resistors are independent, and the voltage-controlled current sources at the inputs meet stringent low-noise requirements by reducing the number of amplifiers.

Using more amplifiers to reduce noise is a two-step process. First, assume that the random noise sources of the amplifiers are independent of each other. Further, assume that the noise follows a Gaussian distribution. Using N amplifiers and tripling the resistors reduces the noise to 1/ when averaging the outputs of a classic voltage amplifier (Reference 2). The internal architecture of the AD8553 allows only N+1 resistors to be used while operating with an almost unlimited number of ICs in parallel. Connecting the internal voltage and current sources makes it easy to parallelize by connecting more ICs at their respective input pins (Figure 1). Microvolt input voltage biases are harmless if combined with the parallel input pins of several ICs, because the output resistance of the voltage-to-current converter is theoretically infinite.
 

The result of the network with N parallel input terminals is N(VINP–VINN)/(2R1) or N times the output current of a single IC. Only one of the N IC current and voltage terminals can be used. The terminal feedback resistor is R2/N, where R2 is the expected voltage gain AV value of the single IC. Since the main source of amplifier noise is the input terminal, it is assumed that the random device standard deviation of the output current of the N parallel voltage-to-current converters is σNI=σI×  , where σI is the random device standard deviation of the output current of the voltage-to-current converter. These results are different from those in Reference 2, where the authors achieved noise reduction by multi-voltage averaging. On the other hand, the determining component of the current in the common-mode output of the voltage-to-current converter of Figure 1 is N times that of a single IC. The following formula calculates the RSNR (relative signal-to-noise ratio), which defines the output current that exceeds the output noise standard deviation: RSNRN=(N×I)/(σI× )= ×RSNR1. In practice, this means that the circuit noise is reduced to 1/ of that of a single IC  .