AD604ARSZ-R7 datasheet(18/32 Pages) AD

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AD604

Rev. D | Page 18 of 32

APPLICATIONS INFORMATION

The basic circuit in Figure 43 shows the connections for one

channel of the AD604. The signal is applied at Pin 5. RGN is

normally 0, in which case the preamplifier is set to a gain of 5

(14 dB). When FBK1 is left open, the preamplifier is set to a

gain of 10 (20 dB) and the gain range shifts up by 6 dB. The ac

coupling capacitors before −DSX1 and +DSX1 should be selected

according to the required lower cutoff frequency. In this example,

the 0.1 μF capacitors, together with the 175 Ω seen looking into

each of the DSX input pins, provide a −3 dB high-pass corner of

about 9.1 kHz. The upper cutoff frequency is determined by the

bandwidth of the channel, which is 40 MHz. Note that the signal

can be simply inverted by connecting the output of the preamplifier

to −DSX1 instead of +DSX1; this is due to the fully differential

input of the DSX.

AD604

VGN

RGN

+2.5V

+5V

–5V

OUT

–DSX1

+DSX1

PAI1

FBK1

PAO1

COM1

COM2

PAI2

FBK2

PAO2

+DSX2

–DSX2

VGN1

VREF

VPOS

GND1

OUT1

VNEG

VPOS

GND2

OUT2

VOCM

VGN2

0.1µF

VIN

0.1µF

500Ω

Figure 43. Basic Connections for a Single Channel

In Figure 43, the output is ac-coupled for optimum

performance. For dc coupling, as shown in Figure 52, the capacitor

can be eliminated if VOCM is biased at the same 3.3 V common-

mode voltage as the analog-to-digital converter, AD9050.

VREF requires a voltage of 1.25 V to 2.5 V, with between 40 dB/V

and 20 dB/V gain scaling, respectively. Voltage VGN controls

the gain; its nominal operating range is from 0.25 V to 2.65 V

for 20 dB/V gain scaling and 0.125 V to 1.325 V for 40 dB/V

scaling. When VGNx is grounded, the channel powers down

and disables its output.

COM1 is the main signal ground for the preamplifier and needs

to be connected with as short a connection as possible to the input

ground. Because the internal feedback resistors of the preamplifier

are very small for noise reasons (8 Ω and 32 Ω nominally), it is

of utmost importance to keep the resistance in this connection

to a minimum. Furthermore, excessive inductance in this

connection can lead to oscillations.

Because of the ultralow noise and wide bandwidth of the

AD604, large dynamic currents flow to and from the power

supply. To ensure the stability of the part, extreme attention to

supply decoupling is required. A large storage capacitor in

parallel with a smaller high frequency capacitor connected at

the supply pins, together with a ferrite bead coming from the

supply, should be used to ensure high frequency stability.

To provide for additional flexibility, COM1 can be used to

disable the preamplifier. When COM1 is connected to VP, the

preamplifier is off, yet the DSX portion can be used independently.

This may be of value when cascading the two DSX stages in the

AD604. In this case, the first DSX output signal with respect to

noise is large and using the second preamplifier at this point

would waste power (see Figure 44).

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