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OPA627: why the Input voltage noise density at 1kHz of OPA627 is smaller than OP07 but the input voltage noise of OPA627 is bigger than OP07?

Part Number: OPA627
Other Parts Discussed in Thread: OP07


1. the data is as below, why?

                   the Input voltage noise density at 1kHz      input voltage noise 

OPA627       5.2nV/sqrt(Hz)                                              0.6uV

OP07           9.8nV/sqrt(Hz)                                              0.38uV

2. Besides, can we use the Input voltage noise density to calculate input voltage noise?

  • Hi Justifice,

    what you call "input voltage noise" is the very low frequency noise in the 0.1Hz...10Hz band. This must not be confused with the "input voltage noise density" at 1kHz.

    The OP07 and the OPA627 are totally different OPAmp for very different applications. The OP07 contains an input stage with BJT (bipolar junction transistors) while the OPA627 has JFETs. Both transistor types show different noise performance at 1kHz and very low frequencies (0.1Hz...10Hz band). The OPA627 shows a higher 1/f input voltage noise (figure 1 of datasheet) compared to the OP07 which makes the noise increasing more rapidely at the very low frequencies, while the OPA627 shows a lower input voltage noise density at the higher frequencies.

    Unfortunately, TI does not show the input voltage noise density curve of OP07 in its datasheet. But this curve can be seen in the datasheets of other manufacturers of OP07.


  • Adding to Kai's explanation, there are typically two regions of the input voltage noise spectral density in most amplifiers: a low frequency (1/f) and higher frequency (broadband) noise.  In bipolar op amps (like OP07) 1/f corner is lower frequency than in JFET op amps like OPA627 while in CMOS op amps 1/f corner frequency is at the highest (see below). Thus, when you talk about the input voltage noise spectral density at 1kHz you mostly refer to the high frequency noise while the input voltage noise limit is typically an integrated low frequency (1/f) noise between from 0.1Hz to 10Hz - these are completely different things and the reason why higher OP07 input voltage noise at 1kHz has not bearing on its lower 1/f noise.

    Below please find the equations for calculating a total noise based on 1/f and broadband parts of the noise - see below.

    Also, below please review app note discussing trade-offs between different technologies used to fabricate op amps: