Autozeroed amplifier with halved noise needs few components

The Analog Devices (www.analog.com) 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.



Caption

Figure 1 The unique architecture of the AD8553 instrumentation amplifier, incorporating an input-voltage-to-current converter, allows 50% noise reduction with fewer components.

Click here for Illustrations:

Figure 1