REF5045 datasheet(36/62 Pages) TI2

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AVDD

OPA+

VCM

1 nF

0.2

10 µF

1 k

1 µF

THS4281

1 k

1 µF

OPA333

20 k

1 µF

VIN+

VIN-

OPA350

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 Typical Applications

11.2.1 DAQ Circuit for a 1-µs, Full-Scale Step Response

Figure 65. DAQ Circuit for a 1-µs, Full-Scale Step Response

11.2.1.1 Design Requirements

Step input signals are common in multiplexed applications when switching between different channels. In a

worst-case scenario, one channel is at the negative full-scale (NFS) and the other channel is at the positive full-

scale (PFS) voltage, in which case the step size is the full-scale range (FSR) of the ADC when the MUX channel

is switched.

Design an application circuit optimized for using the ADS8881 to achieve

•

full-scale step input settling to 18-bit accuracy and

•

INL of < ±2 LSB and

•

maximum specified throughput of 1 MSPS

11.2.1.2 Detailed Design Procedure

The application circuit is shown in Figure 65.

In such applications, the primary design requirement is to ensure that the full-scale step input signal settles to 18-

bit accuracy at the ADC inputs. This condition is critical to achieve the excellent linearity specifications of the

ADC. Therefore, the bandwidth of the antialiasing RC filter must be large enough to allow optimal settling of the

input signal during the ADC acquisition time. The filter capacitor helps reduce the sampling charge injection at

the ADC inputs, but degrades the phase margin of the driving amplifier, thereby leading to stability issues.

Amplifier stability is maintained by the series isolation resistor.

The application circuit in Figure 65 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.

REF5045 Datasheet(PDF) 36 Page - Texas Instruments

REF5045 Datasheet(HTML) 36 Page - Texas Instruments

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