AD8099ACPZ-R2 datasheet(21/26 Pages) AD

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Data Sheet

AD8099

Rev. E | Page 21 of 26

TOTAL OUTPUT NOISE CALCULATIONS AND

DESIGN

To analyze the noise performance of an amplifier circuit,

the individual noise sources must be identified. A user must

then determine if the source has a significant contribution to

overall noise performance of the amplifier. To simplify the

noise calculations, this data sheet works with noise spectral

densities rather than actual voltages to leave bandwidth out

of the expressions (noise spectral density, which is generally

expressed in nV/√Hz, is equivalent to the noise in a 1 Hz

bandwidth).

The noise model shown in Figure 74 has six individual noise

sources: the Johnson noise of the three resistors, the op amp

voltage noise, and the current noise in each input of the amplifier.

Each noise source has its own contribution to the noise at the

output. Noise is generally specified referred to input (RTI), but

it is often simpler to calculate the noise referred to the output

(RTO) and then divide by the noise gain to obtain the RTI noise.

All resistors have a Johnson noise of √(4kBTR), where k is

Boltzmann’s Constant (1.38 × 10−23 J/K), T is the absolute

temperature in Kelvin, B is the bandwidth in Hz, and R is

the resistance in ohms. A simple relationship, which is easy to

remember, is that a 50 Ω resistor generates a Johnson noise of

1 nV√Hz at 25°C. The AD8099 amplifier has roughly the same

equivalent noise as a 50 Ω resistor.

GAIN FROM

"B" TO OUTPUT

= –

GAIN FROM

"A" TO OUTPUT

NOISE GAIN =

NG = 1 +

IN–

VN, R1

VN, R3

IN+

4kTR2

4kTR1

4kTR3

VN, R2

VN2 + 4kTR3 + 4kTR1

R1 + R2

IN+2R32 + IN–2

R1 × R2

+ 4kTR2

R1 + R2

RTI NOISE =

RTO NOISE = NG × RTI NOISE

VOUT

Figure 74. Op Amp Noise Analysis Model

In applications where noise sensitivity is critical, care must be

taken not to introduce other significant noise sources to the

amplifier. Each resistor is a noise source. Attention to design,

layout, and component selection is critical to maintain low

noise performance. A summary of noise performance for the

amplifier and associated resistors can be seen in Table 4.

INPUT BIAS CURRENT AND DC OFFSET

In high noise gain configurations, the effects of output offset

voltage can be significant, even with low input bias currents and

input offset voltages. Figure 75 shows a comprehensive offset

voltage model, which can be used to determine the referred to

output (RTO) offset voltage of the amplifier or referred to input

(RTI) offset voltage.

GAIN FROM

"B" TO OUTPUT

= –

GAIN FROM

"A" TO OUTPUT

NOISE GAIN =

NG = 1 +

IB–

VOS

IB+

OFFSET (RTO) = VOS 1 +

+ IB+ × R3 1 +

– IB– × R2

OFFSET (RTI) = VOS + IB+ × R3 – IB–

R1 × R2

R1 + R2

OFFSET (RTI) = VOS

IF IB+ = IB– AND R3 =

R1 × R2

R1 + R2

VOUT

FOR BIAS CURRENT CANCELLATION:

Figure 75. Op Amp Total Offset Voltage Model

For RTO calculations, the input offset voltage and the voltage

generated by the bias current flowing through R3 are multiplied

by the noise gain of the amplifier. The voltage generated by IB−

through R2 is summed together with the previous offset voltages to

arrive at a final output offset voltage. The offset voltage can also

be referred to the input (RTI) by dividing the calculated output

offset voltage by the noise gain.

As seen in Figure 75, if IB+ and IB– are the same and R3 equals

the parallel combination of R1 and R2, then the RTI offset

voltage can be reduced to only VOS.This is a common method

used to reduce output offset voltage. Keeping resistances low

helps to minimize offset error voltage and keeps the voltage

noise low.

DISABLE PIN AND INPUT BIAS CANCELLATION

The AD8099 DISABLE pin performs three functions; enable,

disable, and reduction of the input bias current. When the

DISABLE pin is brought to within 0.7 V of the positive supply,

the input bias current is reduced by an approximate factor of 60.

However, the input current noise doubles to 5.2 pA/√Hz. Table 5

outlines the DISABLE pin functionality.

Table 5. DISABLE Pin Truth Table

Supply Voltage

±5 V

+5 V

Disable

−5 to +2.4

0 to 2.4

Enable

Open

Low Input Bias Current

4.3 to 5

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