DocID026988 Rev 1

5/12

AN4598

Distortion

12

Independent noise sources (measured in Vrms) can be combined together by taking the

square root of the sum of their squares.

If we consider a positive gain amplification of the output of the microphone (as shown in

Figure 2) and if the noise level generated by the op amp is only half the noise level

generated by the microphone (erms), the resulting overall noise is only 1.12 * erms.

Then, considering the same 20 kHz bandwidth, using an op amp whose equivalent white

noise density is only 28 nV/VHz only increases the noise level by 12 % for the MP23AB02B.

To avoid decreasing significantly the noise level and therefore the signal-over-noise ratio, an

op amp with a white noise of less than 20 nV/√Hz is recommended and less than 10 nV/√Hz

is perfect.

Note that the overall noise level at the amplification output depends greatly on the shape of

the gain stage (cutoff frequencies and filter orders).

Also, it is important when designing a circuit to bear in mind that resistors are a source of

noise and that a resistor, R kΩ, generates a white noise source of 4√(R) nV/√Hz.

So, it is important to keep impedances relatively low.

3

Distortion

The signal distortion on the output of the microphone depends on the acoustic pressure as

specified in the datasheet.

For the MP23AB02B, the signal distortion is less than 0.5 % at 94 dBSPL (which is

equivalent to 1 Pa).

For high acoustic pressure, the distortion level increases. A 10 % distortion level is shown in

Figure 1. The microphone can be used from the noise floor up to the distortion level. This is

its dynamic range.

The signal distortion should be compared with the total harmonic distortion (THD) which is

introduced by the op am. Generally op amps are not limiting for this parameter.

But we should bear in mind that the THD increases when frequency increases, for large

output signals and low load values.

A pure sine waveform on the output of the op amp can calculated using Equation 5.

Equation 5

must be 0.19 V/us. A margin (of at least a factor of two) should be considered on this value

to ensure the smallest distortion of the signal.

2

πf

()

sin