±121339

±121338

±121337

±121336

±121335

±121334

0

20

40

60

80

100

Time (µs)

C048

121405

121406

121407

121408

121409

121410

0

20

40

60

80

100

Time (µs)

C047

16

REF6125, REF6130, REF6133, REF6141, REF6145, REF6150

SBOS747B – MAY 2016 – REVISED AUGUST 2016

www.ti.com

Product Folder Links: REF6125 REF6130 REF6133 REF6141 REF6145 REF6150

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Copyright © 2016, Texas Instruments Incorporated

8.3 Reference Droop Measurements

Many applications, such as event-triggered and multiplexed data-acquisition systems, require the very first

conversion of the ADC to have 18-bit or greater precision. These types of data-acquisition systems capture data

in bursts, and are also called burst-mode, data-acquisition systems. Achieving 18-bit precision for the first sample

is a very difficult using a conventional voltage reference because the voltage reference droop limits the accuracy

of the first few conversions. The REF61xx have an integrated ADC drive buffer that makes sure the reference

droop is less than 1 LSB at 18-bit precision when used with the ADS8881, even at full throughput. Figure 44 and

Figure 45 show the REF61xx output voltage droop when driving the REF pin of the ADS8881 at positive and

negative full-scale inputs, respectively.

REF6150 driving REF pin of ADS8881 operating at 1 MSPS,

positive full-scale input to ADS8881

Figure 44. Output Voltage Droop

REF6150 driving REF pin of ADS8881 operating at 1 MSPS,

negative full-scale input to ADS8881

Figure 45. Output Voltage Droop

Direct measurement of the reference droop to 18-bit accuracy can be a challenging process. Therefore, the plots

in Figure 44 and Figure 45 were obtained by processing the output code of the ADC. The ADC output code is

given by:

C = (Input Voltage / VREF) × 2

N

(2)

If the input voltage is kept constant, VREF is computed by monitoring the ADC output code C. The ADC code

usually has six to seven LSBs of code spread due to the inherent noise of the ADC. In order to measure

reference droop, this noise must be reduced drastically. Noise reduction is done by averaging the output code

multiple times, as described in the next paragraph.