AD605

Rev. E | Page 16 of 20

APPLICATIONS INFORMATION

The basic circuit in Figure 38 shows the connections for one

channel of the AD605 with a gain range of −14 dB to +34.4 dB.

The signal is applied at Pin 3. The ac coupling capacitors before

Pin −IN1 and Pin +IN1 should be selected according to the

required lower cutoff frequency. In this example, the 0.1 μF

capacitors, together with the 175 Ω of 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 amplifier and is

40 MHz.

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8

1

2

3

4

7

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5

VREF

GND1

+IN1

–IN1

VGN1

OUT1

FBK1

VPOS

–IN2

+IN2

GND2

VPOS

FBK2

OUT2

VOCM

VGN2

AD605

VGN

0.1µF

0.1µF

0.1µF

5V

0.1µF

OUT

2.500V

Figure 38. Basic Connections for a Single Channel

As shown in Figure 38, the output is ac-coupled for optimum

performance. In the case of connecting to the 10-bit, 40 MSPS

ADC, AD9050, ac coupling can be eliminated as long as

Pin VOCM is biased by the same 3.3 V common-mode

voltage as the AD9050.

Pin VREF requires a voltage of 1.25 V to 2.5 V, with gain scaling

between 40 dB/V and 20 dB/V, 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 this pin is taken to ground, the channel powers

down and disables its output.

CONNECTING TWO AMPLIFIERS TO DOUBLE THE

GAIN RANGE

Figure 39 shows the two channels of the AD605 connected in

series to provide a total gain range of 96.8 dB. When R1 and R2

are shorts, the gain range is from −28 dB to +68.8 dB with a

slightly reduced bandwidth of about 30 MHz. The reduction in

bandwidth is due to two identical low-pass circuits being

connected in series; in the case of two identical single-pole, low-

pass filters, the bandwidth is reduced by exactly √2. If R1 and

R2 are replaced by open circuits, that is, Pin FBK1 and Pin FBK2

are left unconnected, the gain range shifts up by 28 dB to 0 dB

to 96.8 dB. As previously noted, the bandwidth of each individual

channel is reduced by a factor of 5 to about 8 MHz because the

gain increased by 14 dB. In addition, there is still the √2 reduction

because of the series connection of the two channels that results

in a final bandwidth of the higher gain version of about 6 MHz.

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9

8

1

2

3

4

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6

5

VREF

GND1

+IN1

–IN1

VGN1

OUT1

FBK1

VPOS

–IN2

+IN2

GND2

VPOS

FBK2

OUT2

VOCM

VGN2

AD605

C2

0.1µF

VGN

VIN

R1

5V

OUT

2.500V

C1

0.1µF

C3

0.1µF

C4

0.1µF

C6

0.1µF

R2

C5

0.1µF

Figure 39. Doubling the Gain Range with Two Amplifiers

Two other easy combinations are possible to provide a gain

range of −14 dB to +82.8 dB: make R1 a short and R2 an open,

or make R1 an open and R2 a short. The bandwidth for both of

these cases is dominated by the channel that is set to the higher

gain and is about 8 MHz. From a noise standpoint, the second

choice is the best because by increasing the gain of the first

amplifier, the noise of the second amplifier has less of an impact

on the total output noise. One further observation regarding

noise is that by increasing the gain, the output noise increases

proportionally; therefore, there is no increase in signal-to-noise

ratio. It actually stays fixed.

It should be noted that by selecting the appropriate values of R1

and R2, any gain range between −28 dB to +68.8 dB and 0 dB to

+96.8 dB can be achieved with the circuit in Figure 39. When

using any value other than shorts and opens for R1 and R2, the

final value of the gain range depends on the external resistors

matching the on-chip resistors. Because the internal resistors

can vary by as much as ±20%, the actual values for a particular

gain have to be determined empirically. Note that the two channels

within one part match quite well; therefore, R1 tracks R2 in

Figure 39.

C3 is not required because the common-mode voltage at

Pin OUT1 should be identical to the one at Pin +IN2 and

Pin −IN2. However, because only 1 mV of offset at the output of

the first DSX introduces an offset of 53 mV when the second

DSX is set to the maximum gain of the lowest gain range

(34.4 dB), and 263 mV when set to the maximum gain of the

highest gain range (48.4 dB), it is important to include ac

coupling to get the maximum dynamic range at the output of

the cascaded amplifiers. C5 is necessary if the output signal

needs to be referenced to any common-mode level other than

half of the supply as is provided by Pin OUT2.