REF5045 datasheet(38/62 Pages) TI2

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AVDD

OPA+

VCM

2.2 nF

0.2

10 µF

1 k

1 µF

THS4281

1 k

1 µF

OPA333

20 k

1 µF

VIN+

VIN-

OPA320

AINP

AINM

REF

GND

AVDD

CONVST

Device

Input Driver

Reference Drive Circuit

SAR ADC

OPA+

REF5045

(See Reference Datasheet

for Detailed Pin Configuration)

ADS8881

SBAS547D – MAY 2013 – REVISED AUGUST 2015

www.ti.com

Product Folder Links: ADS8881

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11.2.2 Low-Power DAQ Circuit for Excellent Dynamic Performance at 1 MSPS

Figure 67. DAQ Circuit for Lowest Power and Excellent Dynamic Performance at 1 MSPS

11.2.2.1 Design Requirements

Design an application circuit optimized for using the ADS8881 to achieve

•

ENOB > 17 bits and

•

< 35 mW of total power consumption and

•

maximum specified throughput of 1 MSPS

11.2.2.2 Detailed Design Procedure

The application circuit in Figure 67 shows the schematic of a complete reference driver circuit that generates a

voltage of 4.5 V dc using a single 5-V supply. This circuit is suitable to drive the reference of the ADS8881 at

higher sampling rates up to 1 MSPS. The reference voltage of 4.5 V in this design is generated by the high-

precision, low-noise REF5045 circuit. The output broadband noise of the reference is heavily filtered by a low-

pass filter with a 3-dB cutoff frequency of 160 Hz.

The reference buffer is designed with the THS4281 and OPA333 in a composite architecture to achieve superior

dc and ac performance at a reduced power consumption, compared to using a single high-performance amplifier.

The THS4281 is a high-bandwidth amplifier with a very low output impedance of 1 Ω at a frequency of 1 MHz.

The low output impedance makes the THS4281 a good choice for driving a high capacitive load to regulate the

voltage at the reference input of the ADC. The high offset and drift specifications of the THS4281 are corrected

by using a dc-correcting amplifier (OPA333) inside the feedback loop. The composite scheme inherits the

extremely low offset and temperature drift specifications of the OPA333.

In such applications, the input driver must be low in power and noise as well as able to support rail-to-rail input

and output swing with a single supply. A high amplifier bandwidth is also preferred to help attenuate high-

frequency distortion. However, oftentimes bandwidth and noise are traded off with the power consumption of the

amplifier. This circuit uses the OPA320 as the front-end driving amplifier because this device has a relatively low

noise density of 7 nV/

√Hz for a maximum-specified quiescent current of 1.45 mA per channel.

The noise contribution from the front-end amplifier is band-limited by the 3-dB bandwidth of the RC filter and is

designed to be 1.65 MHz in this application. Again, the component values of the antialiasing filter are carefully

selected to maintain the stability of the input driving amplifiers.

REF5045 Datasheet(PDF) 38 Page - Texas Instruments

REF5045 Datasheet(HTML) 38 Page - Texas Instruments

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