ADA4899-1YCPZ-R2 datasheet(16/20 Pages) AD

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ADA4899-1

Data Sheet

Rev. C | Page 16 of 20

DISABLE PIN OPERATION

The ADA4899-1 DISABLE pin performs three functions:

enable, disable, and reduction of the input bias current. When

the DISABLE pin is brought to within 0.7 V of the positive

supply, the input bias current circuit is enabled, which reduces

the input bias current by a factor of 100. In this state, the input

current noise doubles from 2.6 pA/Hz to 5.2 pA/Hz. Table 7

outlines the DISABLE pin operation.

Table 7. DISABLE Pin Truth Table

Supply Voltage

±5 V

+5 V

Disable

−5 V to +2.4 V

0 V to 2.4 V

Enable

Open

Low Input Bias Current

4.3 V to 5 V

ADA4899-1 MUX

With a true output disable, the ADA4899-1 can be used in

multiplexer applications. The outputs of two ADA4899-1

devices are wired together to form a 2:1 mux. Figure 51 shows

the 2:1 mux schematic.

1MHz

0V TO 5V

+5V

–5V

+5V

–5V

0.1µF

ADA4899-1

2V p-p

15MHz

+5V

–5V

0.1µF

ADA4899-1

0.1µF

2.2µF

0.1µF

2.2µF

2kΩ

1kΩ

50Ω

1.02kΩ

50Ω

VREF = 2.50V

2kΩ

1V p-p

15MHz

DISABLE

AD8137

DISABLE

50Ω

VOUT

Figure 51. ADA4899-1 2:1 Mux Schematic

An AD8137 differential amplifier is used as a level translator

that converts the TTL input to a complementary ±3 V output to

drive the DISABLE pins of the ADA4899-1 devices. The

transient response for the 2:1 mux is shown in Figure 52.

CH1 = 500mV/DIV

CH2 = 5V/DIV

200ns/DIV

Figure 52. ADA4899-1 2:1 Mux Transient Response

CIRCUIT CONSIDERATIONS

Careful and deliberate attention to detail when laying out the

ADA4899-1 board yields optimal performance. Power supply

bypassing, parasitic capacitance, and component selection all

contribute to the overall performance of the amplifier.

PCB Layout

Because the ADA4899-1 can operate up to 600 MHz, it is

essential that RF board layout techniques be employed. All

ground and power planes under the pins of the ADA4899-1

should be cleared of copper to prevent the formation of parasitic

capacitance between the input pins to ground and the output

pins to ground. A single mounting pad on a SOIC footprint can

add as much as 0.2 pF of capacitance to ground if the ground

plane is not cleared from under the mounting pads. The low

distortion pinout of the ADA4899-1 reduces the distance

between the output and the inverting input of the amplifier.

This helps minimize the parasitic inductance and capacitance

of the feedback path, which reduces ringing and second harmonic

distortion.

Power Supply Bypassing

Power supply bypassing for the ADA4899-1 has been optimized

for frequency response and distortion performance. Figure 40

shows the recommended values and location of the bypass

capacitors. Power supply bypassing is critical for stability,

frequency response, distortion, and PSR performance. The

0.1 μF capacitors shown in Figure 40 should be as close to the

supply pins of the ADA4899-1 as possible. The electrolytic

capacitors should be directly adjacent to the 0.1 μF capacitors.

The capacitor between the two supplies helps improve PSR and

distortion performance. In some cases, additional paralleled

capacitors can help improve frequency and transient response.

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