7.3 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 REF60xx have an integrated ADC drive buffer that makes sure the

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

Figure 7-5 and Figure 7-6 show the REF60xx output voltage droop when driving the REF pin of the ADS8881 at

positive and negative full-scale inputs, respectively.

121405

121406

121407

121408

121409

121410

0

20

40

60

80

100

Time (µs)

C047

REF6050 driving REF pin of ADS8881 operating at 1MSPS,

positive full-scale input to ADS8881

Figure 7-5. Output Voltage Droop

±121339

±121338

±121337

±121336

±121335

±121334

0

20

40

60

80

100

Time (µs)

C048

REF6050 driving REF pin of ADS8881 operating at 1MSPS,

negative full-scale input to ADS8881

Figure 7-6. Output Voltage Droop

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

in Figure 7-5 and Figure 7-6 were obtained by processing the output code of the ADC. The ADC output code is

given by:

C = (Input Voltage / VREF) × 2N

(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.

REF60

SBOS708C – MAY 2016 – REVISED APRIL 2026

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