ADA4511-2ARMZ-R7 datasheet(26/28 Pages) AD

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Data Sheet

ADA4511-2

APPLICATIONS INFORMATION

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Rev. A | 26 of 28

Figure 88. Low-Pass Filter: Gain vs. Frequency

FEEDBACK COMPONENTS

When feedback resistors are used to set up gain, care must be

taken to ensure that the pole formed by the resistors and the para-

sitic capacitance at the inverting input does not degrade stability.

If the pole formed is near the required crossover frequency of the

amplifier, then it negatively impacts the stability.

In general, if the parasitic pole lies within the closed-loop bandwidth

of the amplifier, add a capacitor in parallel with the RF to introduce

a zero that has a frequency close to the frequency of the pole to

improve stability.

For a more detailed discussion, refer to the Analog Dialogue article:

The Truth About Voltage Feedback Resistors.

PRECISION BUFFER

The overall precision of high-resolution systems with ADCs and

digital-to-analog converters (DACs) depends on the accuracy, sta-

bility, and drive capability of the voltage reference of the system.

Typically, the best performance requires a costly external reference,

because on-chip references and buffers often have poor perform-

ance or insufficient drive.

With its low-noise specifications, the ADA4511-2 can be used to

preserve the accuracy of the chosen reference for successive

approximation register (SAR) ADC reference inputs. The Analog

Dialogue article: Voltage Reference Design for Precision Succes-

sive-Approximation ADCs details several considerations and how

to compute for noise from the reference circuit to ensure that ADC

performance is not affected.

DAC outputs that drive real world sensors also depend on the

accuracy of the reference voltage. The low VOS, ΔVOS/ΔT, IB, en

p-p, and very high linearity, in combination with the fast settling time

and slew rate, make the ADA4511-2 an ideal fit for an output DAC

buffer.

RECOMMENDED POWER SOLUTION

Analog Devices, Inc., has a wide range of power management

products that meet the requirements of most high performance

signal chains. For a dual-supply application, the ADA4511-2 may

need as high as ±20 V supply. Low dropout (LDO) linear regulators

such as the LT3042 for the positive supply and the LT3093 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. Table 8 shows the list of the recommended power

management devices for the ADA4511-2.

Table 8. Recommended Power Management Devices

Product

Description

ADP5070

DC-to-DC switching regulator with independent

positive and negative outputs

LT3032

Dual 150 mA positive/negative low-noise LDO

linear regulator

LT3093

−20 V, 200 mA, ultra-low noise, ultra-high PSRR

negative linear regulator

LT3042

20 V, 200 mA, ultra-low noise, ultra-high PSRR RF

linear regulator

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

to each of the power supply pins of the ADA4511-2 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.

LAYOUT GUIDELINES

The ADA4511-2 has extremely high impedance inputs. Shunt impe-

dances from leakage resistance and parasitic capacitance in the

PCB layout can severely degrade the performance of the low bias

input. Protect against parasitic leakage currents by using guarding

techniques to reduce the voltage gradient seen by the input node.

Physically, a guard is a low impedance conductor that surrounds

a high impedance node and is driven to the voltage of that node.

It serves to buffer leakage by diverting the leakage away from

the sensitive node and into the low impedance guard. Remove

the solder mask from the guard traces to guard against surface

leakage due to contamination. Place any input resistors close to

the ADA4511-2 inputs to avoid interaction with trace parasitics. If

one of the channels is not in use, connect the input to a voltage

that is within the linear range of the channel to avoid overdrive

conditions that can interfere with other channels. Leave the output

unconnected. Place decoupling capacitors, such as 0.1 µF, near the

ADA4511-2. Larger capacitors, such as 10 µF, can be used farther

away from the op amp.

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