Data Sheet

AD8337

Rev. D | Page 19 of 28

referenced design, the VGAIN range is from −0.7 V to +0.7 V with the

most linear-in-dB section of the gain control between −0.6 V and +0.6

V. In the center 80% of the VGAIN range, the gain error is typically less

than ±0.2 dB. The gain control voltage can be increased or decreased to

the positive or negative rails without gain foldover.

e gain scaling factor (gain slope) is designed for 20 dB/V. This relatively

low slope ensures that noise on the GAIN input is not unduly

amplified. Because a VGA functions as a multiplier, it is important that

the GAIN input does not inadvertently modulate the output signal with

unwanted noise. Because of its high input impedance, a simple low-

pass filter can be added to the GAIN input to filter unwanted noise.

OUTPUT STAGE

The output stage is a Class AB, voltage-feedback, complementary emitter-

follower with a fixed gain of 18 dB, similar to the preamplifier in speed

and bandwidth. Because of the ac-beta roll-off of the output devices

and the inherent reduction in feedback beyond the −3 dB bandwidth,

the impedance looking into the output pin of the preamp and output

stages appears to be inductive (increasing impedance with increasing

frequency). The high speed output amplifier used in the

AD8337 can

drive large currents, but its stability is susceptible to capacitive

loading. A small series resistor mitigates the effects of capacitive

loading (see the Applications Information section).

ATTENUATOR

The input resistance of the VGA attenuator is nominally 265 Ω. For

200 Ω, the effective preamplifier load is approximately 114 Ω. The

attenuator is composed of eight 3.01 dB sections for a total

attenuation range of −24.08 dB. Following the attenuator is a fixed

gain amplifier with 8× (18.06 dB) gain. Because of this relatively low

gain, the output offset is kept well below 20 mV over temperature; the

offset is largest at maximum gain when the preamplifier offset is

amplified. The VCOM pin defines the common-mode reference for

the output, as shown in Figure 65.

SINGLE-SUPPLY OPERATION AND AC COUPLING

If the

AD8337 is to be operated from a single 5 V supply, the bias

supply for VCOM must be a very low impedance 2.5 V reference,

especially if dc coupling is used. If the device is dc-coupled, the VCOM

source must be able to handle the preamplifier and VGA dynamic load

currents in addition to the bias currents.

When ac coupling the preamplifier input, a bias network and

bypass capacitor must be connected to the opposite polarity input

pin. The bias generator for the VCOM pin must provide the

dynamic current to the preamplifier feedback network and the

VGA attenuator. For many single 5 V applications, a reference, such

as the ADR391, and a good op amp provide an adequate VCOM

source if a 2.5 V supply is unavailable.

NOISE

The total input referred voltage and current noise of the positive

input of the preamplifier are 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, the VGA noise referred all the way to the preamp input is

about 1.3 nV/Hz. From this, it is determined 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 approximately 1.55 nV/Hz.

Equation 2 shows the calculation that determines the output

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

where:

1

FB

R

2

FB

R

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

8×, the noise simplifies to

Hz

nV

35

8)

(1.9

8)

2(1.29

16)

(1.75

2

2

2













(1)

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

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 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.

2

)

VGA

A

n

(e

2

)

VGA

A

n

(e

2

)

VGA

A

FB1

R

FB2

R

n

(e

2

)

S

R

n

(i

2

)

t

A

n

(e

2

t

A

S

R

n

e

n

e

VGA

R

R

PrA

PrA

out

FB2

FB1







































)

(

(2)