ADA4625-2ARDZ-R7 datasheet(32/35 Pages) AD

32 / 35 page

ADA4625-1/ADA4625-2

Data Sheet

Rev. A | Page 32 of 35

RECOMMENDED POWER SOLUTION

Analog Devices has a wide range of power management

products to meet the requirements of most high performance

signal chains.

For a dual-supply application, the ADA4625-1 typically needs a

±15 V supply. Low dropout (LDO) linear regulators such as the

ADP7118 or the ADP7142 for the positive supply and the

ADP7182 for the negative supply help improve the PSRR at

high frequency and generate a low noise power rail. In addition,

if a negative supply is not available, the ADP5070 can generate

the negative supply from a positive supply. Figure 108 shows an

example of this power solution configuration for the ADA4625-1.

ADP5070

ADP7118

ADP7182

+12V

+16V

+15V

–15V

–16V

Figure 108. Power Solution Configuration for the ADA4625-1

Table 9. Recommended Power Management Devices

Product

Description

ADP5070

DC-to-dc switching regulator with independent

positive and negative outputs

ADP7118

20 V, 200 mA, low noise, CMOS LDO linear regulator

ADP7142

40 V, 200 mA, low noise, CMOS LDO linear regulator

ADP7182

−28 V, −200 mA, low noise, linear regulator

It is recommended to use a low ESR, 0.1 μF bypass capacitor close

to each power supply pins of the ADA4625-1 and ground to

reduce errors coupling in from the power supplies. For noisy

power supplies, place an additional 10 μF capacitor in parallel

with the 0.1 μF for better performance.

INPUT OVERVOLTAGE PROTECTION

The ADA4625-1 has internal protective circuitry that allows

voltages as high as 0.2 V beyond the supplies to be applied at the

input of either terminal without causing damage. For higher

input voltages, a series resistor is necessary to limit the input

current. Determine the resistor value by

(VIN − VS)/RS ≤ 20 mA

where:

VIN is the input voltage.

VS is the voltage of either V+ or V−.

RS is the series resistor.

With a very low bias current of <5.5 nA up to 125°C, higher

resistor values can be used in series with the inputs. A 500 Ω

resistor protects the inputs from voltages as high as 10 V beyond

the supplies and adds less than 2.75 µV to the offset. However,

note that the added series resistor (RS) may increase the overall

noise and lower the bandwidth due to the addition of a pole

introduced by RS and the input capacitor of the amplifier.

DRIVING CAPACITIVE LOADS

The inherent output resistance of the op amp combined with a

capacitive load forms an additional pole in the transfer function

of the amplifier. Adding capacitance to the output of any op amp

results in additional phase lag. This lag reduces stability and

leads to overshoot or oscillation, which is a common situation

when an amplifier is used to drive the input of switched

capacitor analog-to-digital converters (ADCs).

The ADA4625-1 has a high phase margin and low output

impedance and is capable of directly driving a capacitive load

up to 1 nF with no external compensation at unity-gain without

oscillation.

For other considerations and various circuit solutions, see the

Ask the Applications Engineer-25, Op Amps Driving Capacitive

Loads Analog Dialogue article.

THERMAL MANAGEMENT

The ADA4625-1 can operate with up to a 36 V supply voltage

with a typical 4 mA quiescent current. Heavy loads increase

power dissipation and raise the chip junction temperature.

The maximum safe power dissipation for the ADA4625-1 is

limited by the associated rise in junction temperature (TJ) on

the die. Two conditions affect TJ: power dissipation (PD) of the

device and ambient temperature (TA) surrounding the package.

This relationship is shown in Equation 11.

TJ = PD × θJA + TA

(11)

where θJA is the thermal resistance between the die and the

ambient environment. The total power dissipation in the

amplifier is the sum of the power dissipated in the output stage

plus the quiescent power. Power dissipation for the sourcing

current is shown in Equation 12, where VSY is the total supply

voltage (V+) – (V−).

PD = VSY × ISY + ((V+) − VOUT)IOUT

(12)

Replace ((V+) − VOUT) in Equation 12 with ((V−) − VOUT) when

sinking current.

For symmetrical supplies with a ground referenced load, use the

following equation to calculate the average power for the amplifier

processing sine signal.

(

)













−













PEAK

)

SINE

AVG,

(13)

where VPEAK is the peak value of a sine wave output voltage.

The specified thermal resistance θJA of the ADA4625-1/ADA4625-2

is 52.8°C/W. A good PCB layout and an external heat sink can

improve thermal performance by reducing junction to ambient

temperature.

The ADA4625-1/ADA4625-2 features an exposed pad that

floats internally to provide the maximum flexibility and ease of

use. Solder the exposed pad to the PCB board GND, or the V+

or V− plane for best thermal transfer. Where thermal heating is not

an issue, the exposed pad can be left floating.

Incorporate the use of thermal vias or heat pipes into the design

of the mounting pad for the exposed pad to lower the overall θJA.

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