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AD8337

Rev. A | Page 20 of 28

NOISE

()

)

VGA

)

VGA

Rfb2

)

VGA

Rfb1

Rfb2

Rfb1

)

PrA

)

PrA

out

−

(1)

The total input-referred voltage and current noise of the positive

input of the preamplifier is about 2.2 nV/√Hz and 4.8 pA/√Hz.

The VGA output referred noise is about 21 nV/√Hz at low gains.

This result is divided by the VGA fixed gain amplifier gain of 8×

and results in a voltage noise density of 2.6 nV/√Hz referred to

the VGA input. This value includes the noise of the VGA gain

setting resistors as well. If this voltage is again divided by the

preamp gain of 2, then the VGA noise referred all the way to the

preamp input is about 1.3 nV/√Hz. From this, we can determine

that the preamplifier, including the 100 Ω gain setting resistors,

contributes about 1.8 nV/√Hz. The two 100 Ω resistors

contribute 1.29 nV/√Hz each at the output of the preamp.

With the gain resistor noise subtracted, the preamplifier noise

is about 1.55 nV/√Hz.

Equation 1 shows the calculation that determines the output

referred noise at maximum gain (24 dB or 16×):

•

At = total gain from preamp input to VGA output;

•

RS = source resistance;

•

en − PrA = input-referred voltage noise of the preamp;

•

in − PrA = current noise of the preamp at the INPP pin;

•

en − Rfb1 = voltage noise of Rfb1; en-Rfb2 = voltage noise of Rfb2;

•

en − VGA = input-referred voltage noise of VGA (low gain,

output-referred noise divided by a fixed gain of 8×).

Assuming RS = 0, RFB1 = RFB2 = 100 Ω, At = 16, AVGA = 8, the noise

simplifies to

(

)(

) (

) =

−

out

(2)

Dividing the result by 16 gives the total input-referred noise

with a short-circuited input as 2.2 nV/√Hz. When the

preamplifier is used in the inverting configuration with the

same RFB1 and RFB2 = 100 Ω as above, then en − out does not

change. However, because the gain dropped by 6 dB, the input-

referred noise increases by a factor of 2 to about 4.4 nV/√Hz.

The reason for this increase is that the noise gain to the output of

all the noise generators stays the same, yet the preamp in the

inverting configuration has a gain of (−1) compared to the (+2)

in the noninverting configuration; this increases the input

referred noise by 2.

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