AD604

REV. 0

–11–

12

11

10

9

8

1

2

3

4

7

6

5

13

16

15

14

24

23

22

21

20

19

18

17

AD604

–DSX1

+DSX1

PAI1

FBK1

PAO1

COM1

COM2

PAI2

FBK2

PAO2

+DSX2

–DSX2

VGN1

VREF

VPOS

GND1

OUT1

VNEG

VNEG

VPOS

GND2

OUT2

VOCM

VGN2

Figure 38. Shutdown of Preamplifiers Only

Differential Ladder (Attenuator)

The attenuator before the fixed gain amplifier of the DSX is

realized by a differential seven-stage R-1.5R resistive ladder net-

work with an untrimmed input resistance of 175

Ω single-ended

or 350

Ω differentially. The signal applied at the input of the

ladder network (Figure 39) 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. A unique circuit technique is used to

interpolate continuously between the tap points, thereby provid-

ing continuous attenuation from 0 to –48.36 dB. You can think

of the ladder network together with the interpolation mechanism

as a voltage-controlled potentiometer.

Since the DSX is a single-supply circuit, some means of biasing

its inputs must be provided. Node MID together with the

VOCM buffer performs this function. Without internal biasing,

the user would have had 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. It should be made clear again that the input to the

DSX is still fully differential if driven differentially, i.e., pins

+DSX and –DSX see the same signal but with opposite polarity

(see Differential Input VGA Application). What changes is the

load as seen by the driver; it is 175

Ω when each input is driven

single ended, but 350

Ω when driven differentially. This can be

easily explained when thinking of the ladder network as just two

175

Ω resistors connected back-to-back with the middle node,

MID, being biased by the VOCM buffer. A differential signal

applied between nodes +DSX and –DSX will result in zero cur-

rent into node MID, but a single-ended signal applied to either

input +DSX or –DSX while the other input is ac grounded, will

cause the current delivered by the source to flow into the

VOCM buffer via node MID.

The ladder resistor value of 175

Ω was chosen to provide the

optimum balance between the load driving capability of the

preamplifier and the noise contribution of the resistors. One fea-

ture of the X-AMP architecture is that the output referred noise

is constant versus gain over most of the gain range. This can be

easily explained by looking at Figure 39 and observing 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. Since there are two attenuators, the overall noise con-

tribution 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 rest

of the DSX circuit components contribute another 1.20 nV/

√Hz

which together with the attenuator produces 1.8 nV/

√Hz of

total DSX input referred noise.

AC Coupling

As already mentioned, the DSX portion of the AD604 is a

single-supply circuit and therefore its inputs need to be ac

coupled to accommodate ground-based signals. External

capacitors C1 and C2 in Figure 35 level shift the ground refer-

enced preamplifier output from ground to the dc value estab-

lished by VOCM (nominal 2.5 V). C1 and C2, together with

the 175

Ω looking into each of DSX inputs (+DSX and –DSX),

will act as high pass filters with corner frequencies depending on

the values chosen for C1 and C2. For example, if C1 and C2

are 0.1

µF, then together with the 175 Ω input resistance seen

into each side of the differential ladder of the DSX, a –3 dB high

pass corner at 9.1 kHz is formed.

If the AD604 output needs to be ground referenced, then an-

other ac coupling capacitor will be required for level shifting.

This capacitor will also eliminate any dc offsets contributed by

the DSX. With a nominal load of 500

Ω and a 0.1 µF coupling

capacitor, this adds a high pass filter with –3 dB corner fre-

quency at about 3.2 kHz.

The choice for all three of these coupling capacitors depends on

the application. They should allow the signals of interest to pass

unattenuated, while at the same time they can be used to limit

the low frequency noise in the system.

R

–6.908dB

R

1.5R

1.5R

R

R

–13.82dB

R

1.5R

1.5R

R

–20.72dB

R

1.5R

1.5R

R

–27.63dB

R

1.5R

1.5R

R

–34.54dB

R

1.5R

1.5R

R

–41.45dB

R

1.5R

1.5R

R

–48.36dB

1.5R

1.5R

175

Ω

175

Ω

+DSX

MID

–DSX

NOTE: R = 96

Ω

1.5R = 144

Ω

Figure 39. R–1.5R Dual Ladder Network.