+

GND

Temp

Trim

+

5-V Power Supply

Temp

REF5045

1 µF

200

PŸ

1

NŸ

10 µF

1 µF

OPA333

1

NŸ

1 µF

20

NŸ

1 µF

To REF pin

of ADC

200

PŸ

10 µF

THS4281

Copyright © 2016, Texas Instruments Incorporated

20

REF6225, REF6230, REF6233, REF6241, REF6245, REF6250

SBOS748 – SEPTEMBER 2016

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Product Folder Links: REF6225 REF6230 REF6233 REF6241 REF6245 REF6250

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Copyright © 2016, Texas Instruments Incorporated

8.3 Feature Description

8.3.1 Integrated ADC Drive Buffer

Many ADC data sheets specify a few microamps of average current draw from the REF pin. Almost all voltage

references provide these few microamps of average current; but not all voltage references are practical for

driving a high-resolution, high-throughput SAR ADC because the peak current drawn can be very high when the

capacitors are switched on the REF pin. The worst-case demand for the voltage reference is during a burst-mode

conversion, when the ADC is idle for a very long time, before a conversion is initiated, and the first sample

converted is expected to be precise. Usually, a large capacitor is connected between the REF pin and ground pin

(or sometimes between the REFP and REFM pins) of the ADC to smoothen the current load and reduce the

burden on the voltage reference. The voltage reference must then be capable of providing the average current

required to completely charge the reference capacitor, but without causing the reference voltage to droop

significantly. Most voltage references lack the ability to completely charge the reference capacitor, and settle

when the binary-weighted capacitors are being switched onto the REF pin because of the large output

impedance. Usually, voltage references have output impedances in the range of 10's of ohms at frequencies

higher than 100 Hz. The output voltage of the voltage reference must be buffered with a low output impedance

(usually high bandwidth) amplifier to achieve excellent linearity and distortion performance.

The key amplifier specifications to be considered when designing a reference buffer for a high-precision ADC

are: low offset, low drift, wide bandwidth, and low output impedance. While it is possible to select an amplifier

that sufficiently meets all these requirements, the amplifier comes at a cost of excessive power consumption. For

example, the OPA350 is a 38-MHz bandwidth amplifier with a maximum offset of 0.5 mV, and low offset drift of 4

µV/ºC, but consumes a quiescent current of 5.2mA. This is because (from an amplifier design perspective) offset

and drift are dc specifications, whereas bandwidth, low output impedance, and high capacitive drive capability

are high-frequency specifications. Therefore, achieving all the performance in one amplifier requires power.

However, a more efficient design to meet the low power budget is to use a composite reference buffer, which

uses an amplifier with superior high-frequency specifications in the feedback loop of a dc precision amplifier to

get the overall performance at much lower power consumption. Figure 48 shows such a composite amplifier

design with the OPA333 (dc precision amplifier) and THS4281 (high-bandwidth amplifier). This reference buffer

design requires three devices, and a large number of external components. This solution still consumes close to

2 mA of quiescent current.

Figure 48. Composite Amplifier Reference Buffer

The REF62xx family of voltage references have an integrated low output impedance buffer (ADC drive buffer);

therefore, there is no need for an external buffer while driving the REF pin of high-precision, high-throughput

SAR ADCs, as shown in Figure 49. The ADC drive buffer of the REF62xx is capable of replenishing a charge of

70 pC on a 47-µF capacitor in 1 µs, without allowing the voltage on the capacitor to droop more than 1 LSB at

18-bit precision. The REF62xx are trimmed at multiple temperatures in production, achieving a max drift of just 3