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# TIDA-060025: OPA858 noise calculation in TIDA-060025

Part Number: TIDA-060025
Other Parts Discussed in Thread: OPA858

I have a TIA question concerning a noise calculation in the TIDA-060025 reference design document.

I am calculating a different noise current value than the reference design indicates.

The TIDA-060025 reference design implements a TIA front end with a 200+ MHz Bandwidth using an OPA858 device.

Section 2.4.1.3 of the TIDA-060025 document indicates a calculated noise level of 132 nArms for the circuit conditions.

Cs = 2 pF

Cf = .75 pf

Rfeedback = 10K

Q=.95

BW=260 MHz

The TIDA-060025 documents references TI document SBOA122 – Transimpedance Considerations for Hight-Speed Amplifiers.

When I calculate the input referred noise equation using equation 13  in the SBOA122 document I get 78.8nArms

I am not far off but was concerned that I am missing something.

Equation 13 - Ieq = sqrt(Ib^2 + 4kT/RF + (en/RF)^2 + (en*2*pi()*F*Cs)^2/3)

en = 2.5 nV/rtHz

Ib = 1e-15 Amp

RF = 10K

Cs = 2 pF

F = 260 MHz

Ieq = 4.89 pA/rtHz

Irms = 78.8 nArms (over 260 MHz BW)

• Hello Francis,

That equation assumes a fixed Q of 0.707 to make hand calculation easier. This can be used for a good initial approximation, for a more accurate number you can use simulation to get the noise level.

Best,

Hasan Babiker

• Thanks Hasan,

I did use the noise simulation tool and got the same results as TIDA-060025.

I will use the TINA tool moving forward.

thanks,

Frank Kimpel