Analog Devices AD604 ULTRALOW NOISE, DIFFERENTIAL INPUT- DIFFERENTIAL OUTPUT VGA

AD604

Rev. E | Page 21 of 3

At 1 MHz, the attenuation is about −0.2 dB, increasing to −6 dB

at 10 MHz and −28 dB at 100 MHz. Signals less than approximately

1 MHz are not significantly affected.

Figure 49 shows the control voltage vs. the input power at 1 MHz to

the circuit shown in Figure 48; note that the AGC threshold is at

−95 dBm. The output signal level is set to 800 mV p-p by applying

−80 mV to the VSET connector.

AD604

FB–DSX1+DSX1PAI1FBK1PAO1COM1COM2PAI2FBK2PAO2+DSX2–DSX2VGN1VREFVPOSGND1OUT1VNEGVNEGVPOSGND2OUT2VOCMVGN2C50.1µFR2499ΩC6560pFC30.1µFFAIR-RITE#2643000301

00540-048

Figure 48. Modifications of the AGC Amplifier to Create 96 dB of Gain Range

4.54.003.53.01.02.52.01.5CONTROL VOLTAGE (V)1MHz0.5P

(dBm)–100 –90 –80 –70 –60 –50 –40 –30 –20 –10 0 10

00540-049

Figure 49. Control Voltage vs. Input Power of the Circuit in Figure 48

ULTRALOW NOISE, DIFFERENTIAL INPUT-DIFFERENTIAL OUTPUT VGA

Figure 50 shows how to use both preamplifiers and DSXs to

create a high impedance, differential input-differential output

VGA. This application takes advantage of the differential inputs

to the DSXs. Note that the input is not truly differential in the

sense that the common-mode voltage needs to be at g round to

achieve maximum input signal swing. This has largely to do

with the limited output swing capabil ity of the output drivers of

the preamplifiers; they clip around ±2.2 V due to having to drive

an effective load of about 30 Ω. If a different input common-mod e

voltage needs to be accommodated, ac coupling (as in Figure 48)

is recommended. The differential gain range of this circuit runs

from 6 dB to 54 dB, which is 6 dB higher than each individual

channel of the AD604 because the DSX inputs now see twice

the signal amplitude compared with when they are driven

single-ended.

AD604

+5V

–5V

ALLSUPPL Y PINS ARE DECOUPLED AS SHOWN.

+5V

–5V

+5V

VREF

VOUT+

VOUT

IN+

VIN–

–

–DSX1

+DSX1

PAI1

FBK1

PAO1

COM1

COM2

PAI2

FBK2

PAO2

+DSX2

–DSX2

VGN1

VREF

VPOS

GND1

OUT1

VNEG

VPOS

GND2

OUT2

VOCM

VGN2

C13

0.1µF

C12

0.1µF

0.1µF C2

0.1µF

0.1µF C3

0.1µF

453Ω

0.1µF

00540-050

Figure 50. Ultralow Noise, Differential Input-Differential Output VGA

Figure 51 displays the output signals VOUT+ and VOUT− after

a −20 dB attenuator formed between the 453 Ω resistors shown

in Figure 50 and the 50 Ω loads presented by the oscilloscope

plug-in. R1 and R2 are inserted to ensure a nominal load of 500 Ω

at each output. The differential gain of the circuit is set to 20 dB

by applying a control voltage, VGN, of 1 V; the gain scaling is

20 dB/V for a VREF of 2.500 V; the input frequency is 10 MHz,

and the differential input amplitude is 100 mV p-p. The resulting

differential output amplitude is 1 V p-p as can be seen on the

scope photo when reading the vertical scale as 200 mV/div.

100

20ns20mV20mVNOTES1.THE OUTP UT AFTER 10× ATTENUATER FORME DBY453ΩTOGETHER WITH 50Ω OF 7A24 PLUG-IN.–500mVACTUALV

OUT

+500mV

00540-054

At 1 MHz, the attenuation is about −0.2 dB, increasing to −6 dB

at 10 MHz and −28 dB at 100 MHz. Signals less than approximately

1 MHz are not significantly affected.

Figure 49 shows the control voltage vs. the input power at 1 MHz to

the circuit shown in Figure 48; note that the AGC threshold is at

−95 dBm. The output signal level is set to 800 mV p-p by applying

−80 mV to the VSET connector.

AD604

Figure 48. Modifications of the AGC Amplifier to Create 96 dB of Gain Range

Figure 49. Control Voltage vs. Input Power of the Circuit in Figure 48

Figure 50 shows how to use both preamplifiers and DSXs to

create a high impedance, differential input-differential output

VGA. This application takes advantage of the differential inputs

to the DSXs. Note that the input is not truly differential in the

sense that the common-mode voltage needs to be at g round to

achieve maximum input signal swing. This has largely to do

with the limited output swing capabil ity of the output drivers of

the preamplifiers; they clip around ±2.2 V due to having to drive

an effective load of about 30 Ω. If a different input common-mod e

voltage needs to be accommodated, ac coupling (as in Figure 48)

is recommended. The differential gain range of this circuit runs

from 6 dB to 54 dB, which is 6 dB higher than each individual

channel of the AD604 because the DSX inputs now see twice

the signal amplitude compared with when they are driven

single-ended.

AD604

Figure 50. Ultralow Noise, Differential Input-Differential Output VGA

Figure 51 displays the output signals VOUT+ and VOUT− after

a −20 dB attenuator formed between the 453 Ω resistors shown

in Figure 50 and the 50 Ω loads presented by the oscilloscope

plug-in. R1 and R2 are inserted to ensure a nominal load of 500 Ω

at each output. The differential gain of the circuit is set to 20 dB

by applying a control voltage, VGN, of 1 V; the gain scaling is

20 dB/V for a VREF of 2.500 V; the input frequency is 10 MHz,

and the differential input amplitude is 100 mV p-p. The resulting

differential output amplitude is 1 V p-p as can be seen on the

scope photo when reading the vertical scale as 200 mV/div.

Figure 51. Output of VGA in Figure 50 for VGN = 1 V