AD8375

Rev. 0 | Page 14 of 24

BROADBAND OPERATION

The AD8375 uses an open-collector output structure that

requires dc bias through an external bias network. Typically,

choke inductors are used to provide bias to the open-collector

outputs. Choke inductors work well at signal frequencies where

the impedance of the choke is substantially larger than the target

ac load impedance. In broadband applications, it may not be

possible to find large enough choke inductors that offer enough

reactance at the lowest frequency of interest while offering a

high enough self resonant frequency (SRF) to support the

maximum bandwidth available from the device. The circuit in

Figure 36 can be used when frequency response below 10 MHz

is desired. This circuit replaces the bias chokes with bias resistors.

The bias resistor has the disadvantage of a greater IR drop, and

requires a supply rail that is several volts above the local 5 V

supply used to power the device. Additionally, it is necessary

to account for the ac loading effect of the bias resistors when

designing the output interface. Whereas the gain of the AD8375

optimum 150 Ω target load impedance to provide the expected

ac performance depicted in the data sheet. Additionally, to

ensure good output balance and even-order distortion

performance, it is essential that R1 = R2.

5

0.1µF

0.1µF

0.1µF

0.1µF

50Ω

ETC1-1-13

37.5Ω

37.5Ω

5V

AD8375

SET TO

5V

R1

R2

VR

VR

RL

A0 TO A4

Figure 36. Single-Ended Broadband Operation with Resistive Pull-Ups

Using the formula for R1 (Equation 1), the values of R1 = R2

that provide a total presented load impedance of 150 Ω can be

found. The required voltage applied to the bias resistors, VR,

can be found by using the VR formula (Equation 2).

150

75

−

×

=

L

L

R

R

R1

(1)

and

5

10

40

3

+

×

×

=

−

R1

VR

(2)

For example, in the extreme case where the load is assumed to

75 Ω. Using the equation for VR, the applied voltage should be

VR = 8 V. The measured single-tone low frequency harmonic

distortion for a 2 V p-p output using 75 Ω resistive pull-ups is

provided in Figure 37.

–80

–82

–84

–86

–88

–90

–92

–94

–96

0

5

10

15

20

FREQUENCY (MHz)

HD2

HD3

Figure 37. Harmonic Distortion vs. Frequency Using Resistive Pull-Ups

ADC INTERFACING

The AD8375 is a high output linearity variable gain amplifier

that is optimized for ADC interfacing. The output IP3 and noise

floor essentially remain constant vs. the 24 dB available gain

range. This is a valuable feature in a variable gain receiver where

it is desirable to maintain a constant instantaneous dynamic

range as the receiver gain is modified. The output noise density

is typically around 20 nV/√Hz, which is comparable to 14-/16-

bit sensitivity limits. The two-tone IP3 performance of the

AD8375 is typically around 50 dBm. This results in SFDR levels

of better than 86 dB when driving the AD9445 up to 140 MHz.

There are several options available to the designer when using

the AD8375. The open-collector output provides the capability

of driving a variety of loads. Figure 38 shows a simplified

wideband interface with the AD8375 driving a AD9445. The

AD9445 is a 14-bit 125 MSPS analog-to-digital converter with a

buffered wideband input, which presents a 2 kΩ differential

load impedance and requires a 2 V p-p differential input swing

to reach full scale.

0.1µF

0.1µF

50Ω

ETC1-1-13

37.5Ω

37.5Ω

0.1µF

0.1µF

0.1µF

0.1µF

82Ω

82Ω

1µH

5V

1µH

5V

33Ω

33Ω

14

AD9445

14-BIT ADC

AD8375

5

A0 TO A4

L

(SERIES)

L

(SERIES)

VIN+

VIN–

Figure 38. Wideband ADC Interfacing Example Featuring the AD9445