AD604ARSZ-R7 datasheet(15/32 Pages) AD

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AD604

Rev. D | Page 15 of 32

The larger portion of the input referred voltage noise is coming

from the amplifier with 0.63 nV/√Hz. The current noise is

independent of gain and depends only on the bias current in

the input stage of the preamplifier, which is 3 pA/√Hz.

The preamplifier can drive 40 Ω (the nominal feedback resistors)

and the following 175 Ω ladder load of the DSX with low

distortion. For example, at 10 MHz and 1 V at the output, the

preamplifier has less than −45 dB of second and third harmonic

distortion when driven from a low (25 Ω) source resistance.

In applications that require more than 48 dB of gain range, two

AD604 channels can be cascaded. Because the preamplifier has

a limited input signal range, consumes over half (120 mW) of

the total power (220 mW), and its ultralow noise is not necessary

after the first AD604 channel, a shutdown mechanism that

disables only the preamplifier is provided. To shut down the

preamplifier, connect the COM1 pin and/or COM2 pin to the

positive supply; the DSX is unaffected. For additional details,

refer to the Applications Information section.

AD604

–DSX1

+DSX1

PAI1

FBK1

PAO1

COM1

COM2

PAI2

FBK2

PAO2

+DSX2

–DSX2

VGN1

VREF

VPOS

GND1

OUT1

VNEG

VPOS

GND2

OUT2

VOCM

VGN2

Figure 40. Shutdown of Preamplifiers Only

DIFFERENTIAL LADDER (ATTENUATOR)

The attenuator before the fixed gain amplifier of the DSX is

realized by a differential 7-stage R-1.5R resistive ladder network

with an untrimmed input resistance of 175 Ω single-ended or

350 Ω differential. The signal applied at the input of the ladder

network is attenuated by 6.908 dB per tap; thus, the attenuation

at the first tap is 0 dB, at the second, 13.816 dB, and so on, all

the way to the last tap where the attenuation is 48.356 dB

(see Figure 41).

A unique circuit technique is used to interpolate continuously

between the tap points, thereby providing continuous attenuation

from 0 dB to −48.36 dB. The ladder network, together with the

interpolation mechanism, can be considered a voltage-controlled

potentiometer.

Because the DSX circuit uses a single voltage power supply, the

input biasing is provided by the VOCM buffer driving the MID

node (see Figure 41). Without internal biasing, the user would

have to dc bias the inputs externally. If not done carefully, the

biasing network can introduce additional noise and offsets. By

providing internal biasing, the user is relieved of this task and

only needs to ac couple the signal into the DSX. Note that the

input to the DSX is still fully differential if driven differentially,

that is, Pin +DSX and Pin −DSX see the same signal but with

opposite polarity (see the Ultralow Noise, Differential Input-

Differential Output VGA section).

What changes is the load seen by the driver; it is 175 Ω when

each input is driven single-ended but 350 Ω when driven

differentially. This is easily explained by thinking of the ladder

network as two 175 Ω resistors connected back-to-back with

the middle node, MID, being biased by the VOCM buffer. A

differential signal applied between the +DSX and −DSX nodes

results in zero current into the MID node, but a single-ended

signal applied to either input, +DSX or −DSX, while the other

input is ac grounded causes the current delivered by the source

to flow into the VOCM buffer via the MID node.

The ladder resistor value of 175 Ω provides the optimum

balance between the load driving capability of the preamplifier

and the noise contribution of the resistors. An advantage of the

X-AMP architecture is that the output referred noise is constant

vs. gain over most of the gain range. Figure 41 shows that the

tap resistance is equal for all taps after only a few taps away

from the inputs. The resistance seen looking into each tap is

54.4 Ω, which makes 0.95 nV/√Hz of Johnson noise spectral

density. Because there are two attenuators, the overall noise

contribution of the ladder network is √2 times 0.95 nV/√Hz

or 1.34 nV/√Hz, a large fraction of the total DSX noise. The

balance of the DSX circuit components contribute another

1.2 nV/√Hz, which together with the attenuator produces

1.8 nV/√Hz of total DSX input referred noise.

–6.908dB

1.5R

–13.82dB

–20.72dB

–27.63dB

–34.54dB

–41.45dB

–48.36dB

+DSX

MID

–DSX

NOTES

1. R = 96Ω

2. 1.5R = 144Ω

1.5R

175Ω

Figure 41. R-1.5R Dual Ladder Network

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