ADA4817-1ACPZ-R7 datasheet(20/29 Pages) AD

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Data Sheet

ADA4817-1/ADA4817-2

THEORY OF OPERATION

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Rev. H | 20 of 29

Heavy loads increase power dissipation and raise the chip junction

temperature as described in the Absolute Maximum Ratings sec-

tion. Take care not to exceed the rated power dissipation of the

package.

POWER-DOWN OPERATION

The ADA4817-1/ADA4817-2 are equipped with separate power-

down pins (PD) for each amplifier that allow the user the ability to

reduce the quiescent supply current when an amplifier is inactive

from 19 mA to below 2 mA. The power-down threshold levels are

referenced to the +VS pin. The amplifier is enabled when the PD pin

voltage is within 0.9 V of the +VS supply. The amplifier is disabled

when the PD pin voltage is at least 3.5 V from the +VS supply. Table

8 shows the required thresholds for power-down with supplies of

±5 V and 3 V, −2 V, over temperature. If the PD pin is not used,

connect it to the positive power supply to ensure proper startup.

Table 8. PD Pin Control

Supply Voltages

±5 V

+3 V, −2 V

Amplifier Enabled

>4.1 V

>2.1 V

Amplifier Disabled

<1.5 V

<−0.5 V

When the amplifier is powered down with the supplies of +3 V and

−2 V, the PD pin needs to be driven below ground to ensure the

power-down. This may be a problem if a microcontroller is being

used to drive the PD pin. The circuit in Figure 57 can be added to

ensure that the required threshold is met.

Figure 57. Power-Down Circuit

The plot in Figure 58 shows that the PD pin is driven to the positive

rail when the microcontroller logic is high, and to the negative rail

when the microcontroller logic is low. The RF5 and RF6 resistors

must be chosen to be sufficiently high so that minimal current is

drawn by the circuit.

Figure 58. Power-Down Operation

CAPACITIVE FEEDBACK

Due to package variations and pin to pin parasitics between the sin-

gle and the dual models, the ADA4817-2 has a little more peaking

than the ADA4817-1, especially at a gain of 2. The recommended

method to tame the peaking is to place a feedback capacitor across

the feedback resistor. Figure 59 shows the small signal frequency

response of the ADA4817-2 at a gain of 2 vs. CF. At first, no

CF was used to show the peaking; but then two other values of

0.5 pF and 1 pF were used to show how to reduce the peaking

or even eliminate it. If the power consumption is a factor in the

system, using a larger feedback capacitor is acceptable as long as

a feedback capacitor is used across it to control the peaking, as

shown in Figure 59.

However, if power consumption is not an issue, a lower value

feedback resistor, such as 200 Ω, does not require any additional

feedback capacitance to maintain flatness and lower peaking.

Figure 59. Small Signal Frequency Response vs. Feedback Capacitor

(ADA4817-2)

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