NOISE REFERRAL. V nrg Noise due to R g. EXAMPLE 1: The Non-Inverting Amplifier. Finding the noise produced by each source: R g. R f.

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1 To quantiy noise in an op amp circuit, it is customary to ive noise levels in terms o the equivalent input noise. So noise sources in various parts o the circuit are recalculated to the equivalent input noise that would produce the noise in question. The op amp circuit noise model has a noise voltae source or each resistor and one or one o the inputs. (The input voltae noise source may be placed on either input). Additionally, there is a noise current source on each input on the op amp. NOSE EFEAL EXAMPLE : The Nonnvertin Ampliier n evaluatin noise, the placement o and the distribution o input noise between and in are selected to ive the worst case scenario. The conversion ormula or each noise source is derived by indin expressions or resultin rom ) the noise source and ) the equivalent input source. These expressions may then be set equal to each other and a conversion ormula ormed. Two examples ollow: NONNETNG AMPLFE NONNETNG AMPLFE NOSE MODEL n n n Findin the noise produced by each source: Noise due to a resistor: n 4kT k [J/K] T 300 [Kelvin] resistance [Ω] t is easier to visualize the problem i we take each source separately, ind its value, and convert that value to auivalent input noise. The sin o the result is not important since each value will be squared, makin it positive. n Noise due to n n kt 4 This noise will not need to be reerred to the Tom Peck tomzap@eden.com 5/9/99 Pae o 5

2 Noise due to No calculations are required or this case. This noise will not need to be reerred to the input because it is already at the input. Noise due to Since all o the current rom lows throuh to round, the noise due to in terms o voltae is multiplied by. This noise will not need to be reerred to the input. Noise due to The positive input is at round potential due to the path throuh. Under asymptotic conditions, the neative input o the op amp is also at round potential and no current lows into it. So no voltae drop exists across and there is no current low throuh. Thereore, all o the current rom must low throuh and the voltae at will be. To reer the noise to the input: 443 ain o the ampliier Tom Peck tomzap@eden.com 5/9/99 Pae o 5

3 Noise due to n Under asymptotic conditions, the neative input o the op amp is at round potential and no current enters the op amp, so all o the current enerated by n (which is n / ) oes throuh to create a voltae at the put ( n / ). To reer the noise to the input: n 443 n ain o the ampliier 4kT Noise due to n n n this case, the noise at the put due to n is simply n. To reer the noise to the input: n n eq 4kT Findin the Total Noise eerred to the nput o the Nonnvertin Ampliier The noise sources and their input equivalences are listed at riht. To ind the total noise a at the input, sum the squares o each noise component (reerenced to the input) and take the square root o the sum. NOSE SOUCE n n n NOSE EFEED TO NPUT 4kT 4kT 4kT ( ) ( ) a 4kT 4 4 kt kt Tom Peck tomzap@eden.com 5/9/99 Pae 3 o 5

4 EXAMPLE : The nvertin Ampliier NETNG AMPLFE NETNG AMPLFE NOSE MODEL n n n n Noise due to n 4kT n This noise will not need to be reerred to the n Noise due to n n 4kT This noise will not need to be reerred to the Noise due to Moved to positive input To reer the noise to the input: Tom Peck tomzap@eden.com 5/9/99 Pae 4 o 5

5 Noise due to Since the positive input is connected to round, is not a actor. Noise due to Under asymptotic conditions, all o the current rom lows throuh and the voltae at will be. To reer the noise to the input: ( ) Noise due to n n To reer the noise to the input: ( ) n n eq 4 kt Findin the Total Noise eerred to the nput o the nvertin Ampliier The noise sources and their input equivalences are listed at riht. To ind the total noise a at the input, sum the squares o each noise component (reerenced to the input) and take the square root o the sum. NOSE SOUCE n NOSE EFEED TO NPUT 4kT 0 ( ) n 4kT ( ) n 4kT [ ( )] a 4kT 4 kt 4kT Tom Peck tomzap@eden.com 5/9/99 Pae 5 o 5