© 2018 Integrated Device Technology, Inc.

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September 11, 2018

F1240 Datasheet

Figure 53. VGA Output Amplifier - Matched Output Schematic

In

Out

200 Ω

+

-

0

0

Termination Resistors NOT NEEDED:

VGA itself terminates BPF with proper Z = 200 Ω

Total external load = 200 Ω

ALL of the power is delivered to the input of the BPF

Effectively, the

Gain is unchanged!

IDT: ‘Matched’ VGA

200 Ω

200

200 Ω

NO NEED to Adjust Datasheet Values:

Gain with 200 Ω load = 20 dB

True

Gain in-system = 20 dB

Output IP3 with 200 ohm load = +49 dBm

True

IP3O in-system = +49 dBm

200 Ω

Noise Contour

carrier systems. For the first 13 dB of attenuation range, the device has only 2.3dB degradation in noise figure. This is in stark contrast to

standard VGAs like the voltage or current mode devices described earlier. These devices have a linear dB-for-dB degradation in Noise Figure

with increasing attenuation.

Refer to the figure below. It depicts the F1240 driving a matched Anti-Alias Filter which is followed by an ADC with a differential resistive 200

ohm termination. Note that at each point in the system the matching is preserved.

Figure 54. VGA Output Amplifier – Anti-Alias Filter Schematic

C1

L3

C2

L4

C3

L5

C4

VGA_OUT+

VGA_OUT-

L1

L2

CB

5V

200 MHz Discrete Anti-Alias Filter

100

CB

CB

100

CB

V_IN+

V_IN-

Unbuffered

Pipeline ADC

Loss = 2 dB

F1240/1