Bandgap

Voltage

Reference

CFILT = 1 µF

CL = 47 µF

RESR = 5

PŸ

RFILT

Buffer

+

VIN

OUT_S

OUT_F

FILT

VIN

GND_S

GND_F

Power Supply

SS

REF61xx

RLIM = 120

NŸ

EN

GND

REF

+

VIN

R = 1

NŸ

RF = 5

Ÿ

CF = 10 nF

THS4521

AINP

AINN

ADS8881

Copyright © 2016, Texas Instruments Incorporated

R = 1

NŸ

R = 1

NŸ

R = 1

NŸ

RF = 5

Ÿ

Power Supply

17

REF6125, REF6130, REF6133, REF6141, REF6145, REF6150

www.ti.com

SBOS747B – MAY 2016 – REVISED AUGUST 2016

Product Folder Links: REF6125 REF6130 REF6133 REF6141 REF6145 REF6150

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Copyright © 2016, Texas Instruments Incorporated

Reference Droop Measurements (continued)

Figure 46 shows the setup that was used to measure the reference droop. The output ADC code was captured

using a field-programmable gate array (FPGA), and post-processing was done on a personal computer. The

input to the THS4521, and hence in turn to the ADS8881, is a constant dc voltage (close to positive or negative

full-scale because this condition is the worst-case for charge drawn from the REF pin). The dc source must have

extremely low noise. After the REF61xx device is powered up and stable, the FPGA sends commands to the

ADS8881 to capture data in bursts. The ADS8881 is initially in idle mode for 100 ms. The FPGA then sends a

command to the ADS8881 to perform 100 conversions at 1 MSPS. The ADC code corresponding to these 100

conversions (one burst of data) is stored as the first row in a 1000 × 100 dimensional array. This operation is

repeated 1000 times, and the data corresponding to each burst is stored in a new row of the 1000 × 100

dimensional array. Finally, each column in this array is averaged to get a final data-set of 100 elements. This

final data-set now has code spread that is much less than 1 LSB because most of the noise has now been

removed through averaging. This data-set was plotted on a graph with X axis = column number (each column

number corresponds to 1 µs of time because the sampling rate is 1 MSPS), and Y axis = ADC output code to

obtain reference-droop measurements.

Figure 46. Burst-Mode Measurement Setup