1. The AD8429 has total voltage spectral density noise of 45 nV/rtHz at G=1 and f=1kHz, but the AD797 has less than 1 nV/rtHz at f=1kHz. Is it generally true that in amp packages will have greater noise than op amps? Perhaps because of internal thermal noise from resistors or the extra design considerations for in amps packages.

My application for the in amp is to convert a differential signal to a single-ended signal with respect to ground. See Figure 4 in the attached PDF.

It seems that the minimum (total) voltage spectral density for any gain or frequency for the AD8429 is 45 nV/rtHz.

2. For in amps, as compared with op amps, why are there separate parameters for input voltage noise and output voltage noise?

Hi dhhsieh,

Most inamps contain several resistors and opamps and every components contributes to the total noise.

The most popular topology for the instrumentation amplifier is the three-opamp inamp. This architecture is optimized for amplifying small differential signal while rejecting the common mode signal. If your circuit wants to convert the differential signal to single ended signal then you can either use a difference amplifier, instrumentation amplifier or build it using discrete opamps. Note that if you build a discrete difference amplifier, the ratio of the four resistors around the opamp should be very well match to be able to get a good CMRR performance. Total noise of your system will also increase due to the thermal noise of the added resistors and current noise multiplied by the source impedance.

For the inamp, the three opamp inamp model can be classify into two stage: first stage is the input stage or gain stage and the second stage is the differential amplifier stage or output stage which is responsible for rejecting the common mode signal. The noise sources can also be lumped into as single noise source at the input of the Inamp or as two noise sources for the input and output stage of the inamp. The input voltage noise gets multiplied to the gain of the inamp and therefore it dominates the total noise in high gain configuration. On the other hand, output voltage noise dominates the lower gain configuration.

Inamps datasheets often present the total voltage noise RTI (referred to input) as a function of gain. This noise spectral density includes both the input voltage noise and output voltage noise contribution. Please see figure 2 of AD8429 datasheets. To learn more about designing low noise system, you may want to read this article.

Best regards,

Emman