ADA4896-2/ADA4897-1

Rev. 0 | Page 24 of 28

LNA

LNA

AD7982

18-BIT ADC

2.5V

2.5V

4nF

50Ω

50Ω

I

CHANNEL H

CHANNEL A

LO

GENERATION

4

CWI+

CWI–

AD9279

1.5V

1.5V

AD7982

18-BIT ADC

2.5V

2.5V

4nF

50Ω

50Ω

Q

CWQ+

CWQ–

1.5V

1.5V

Φ

Φ

Φ

Φ

ADA4896-2/

ADA4897-1

ADA4896-2/

ADA4897-1

ADA4896-2/

ADA4897-1

ADA4896-2/

ADA4897-1

Figure 54. Using the ADA4896-2/ADA4897-1 as Filters, I-to-V Converters, Current Summers, and ADC Drivers After the I/Q Outputs of the AD9279

LAYOUT CONSIDERATIONS

To ensure optimal performance, careful and deliberate attention

must be paid to the board layout, signal routing, power supply

bypassing, and grounding.

GROUND PLANE

It is important to avoid ground in the areas under and around

the input and output of the ADA4896-2/ADA4897-1. Stray

capacitance created between the ground plane and the input

and output pads of a device are detrimental to high speed

amplifier performance. Stray capacitance at the inverting

input, along with the amplifier input capacitance, lowers the

phase margin and can cause instability. Stray capacitance at

the output creates a pole in the feedback loop. This can reduce

phase margin and can cause the circuit to become unstable.

POWER SUPPLY BYPASSING

Power supply bypassing is a critical aspect in the performance

of the ADA4896-2/ADA4897-1. A parallel connection of capac-

itors from each of the power supply pins to ground works best.

Smaller value capacitors offer better high frequency response,

whereas larger value electrolytics offer better low frequency

performance. Paralleling different values and sizes of capacitors

helps to ensure that the power supply pins are provided a low ac

impedance across a wide band of frequencies. This is important for

minimizing the coupling of noise into the amplifier. This can be

especially important when the amplifier PSR is starting to roll

off—the bypass capacitors can help lessen the degradation in

PSR performance.

Starting directly at the ADA4896-2/ADA4897-1 power supply

pins, the smallest value capacitor should be placed on the same

side of the board as the amplifier, and as close as possible to the

amplifier power supply pin. The ground end of the capacitor

should be connected directly to the ground plane. Keeping the

capacitor’s distance short but equal from the load is important

and can improve distortion performance. This process should

be repeated for the next largest value capacitor.

It is recommended that a 0.1 μF ceramic 0508 case be used. The

0508 case size offers low series inductance and excellent high

frequency performance. A 10 μF electrolytic capacitor should be

placed in parallel with the 0.1 μF capacitor. Depending on the

circuit parameters, some enhancement to performance can be

realized by adding additional capacitors. Each circuit is different

and should be individually analyzed for optimal performance.