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SNLA399 July 2022 OPA607

2.2 Noise

Low voltage noise density and current noise density of OPA607 helps to minimize the noise in the signal chain.

The integrated output voltage noise of the signal chain is 130 µV_RMS which is equivalent to 2.15 pA input referred current noise. Figure 2-3 shows the integrated output noise of signal chain. The input referred current noise of the proposed chain is approximately 24 times smaller than the minimum measurable current, thereby assuring high signal fidelity.

Figure 2-3 Integrated Noise of AFE

The net expression for input referred current spot noise of the signal chain ( i_{n_TIA}) is given by,

Equation 5.

i_{n_T I A} = \sqrt{i_{n}^{2} + {(\frac{e_{n}}{R_{f}})}^{2} + \frac{4 K T}{R_{f}} + \frac{4 K T R_{1}}{R_{f}^{2}} + \frac{4 K T R_{2}}{R_{f}^{2}} + \frac{{(e_{n} 2 π f_{b w} C_{i n})}^{2}}{3}}

Here, f_bw refers to the equivalent noise bandwidth of the signal chain and C_in refers to the net source capacitance. The net source capacitance (C_in) equals to the sum of diode capacitance, common mode and differential capacitance of the op amp. Equivalent noise bandwidth is 1.22 times the circuit bandwidth for a second order filter response.

The output voltage spot noise can be obtained by multiplying the input spot current noise with transimpedance gain R_f. The output spot noise can be multiplied by the square root of noise bandwidth to get the integrated noise. For more details, on noise analysis refer Noise analysis for High Speed Opamps.

For the discussed single operational amplifier based signal chain, current noise of the operational amplifier (i_n) is the dominant noise source as due to the high feedback resistance all other noise terms with a gain factor of 1/R_f becomes negligible.

From Equation 5 with increment in feedback resistance R_f, input referred current noise decreases. This implies a higher R_f results in higher SNR for a given input signal. The change is not very large though, due to the fact that current noise term is dominant which is independent of R_f. For more details on noise analysis of TIA refer Transimpedance Consideration of High Speed Amplifiers.