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LTC2311-16

231116fa

For more information www.linear.com/LTC2311-16

APPLICATIONS INFORMATION

External Reference

The internal reference buffer can also be overdriven from

1.25V to 5V with an external reference at REFOUT as

shown in Figure 13b. In this configuration, REFIN must

be grounded to disable the internal reference buffer. A

55kΩ internal resistance loads the REFOUT pin when

the reference buffer is disabled. To maximize the input

signal swing and corresponding SNR, the LTC6655-5 is

recommendedwhenoverdrivingREFOUT.TheLTC6655-5

offers the same small size, accuracy, drift and extended

temperature range as the LTC6655-4.096. By using a 5V

reference, a higher SNR can be achieved. We recommend

bypassing the LTC6655-5 with a 10μF ceramic capacitor

(X5R, 0805 size) as close as possible to the REFOUT pin.

will affect the accuracy of the output code. Due to the

one-cycle conversion latency, the first conversion result

at the beginning of a burst sampling period will be invalid.

If an external reference is used to buffer/drive the REFOUT

pin, the fast settling LTC6655 reference is recommended.

Figure 13b. LTC2311-16 with an External REFOUT Voltage

VIN

SHDN

VOUT_F

VOUT_S

LTC6655-4.096

LTC2311-16

GND

231116 F13b

5V TO 13.2V

REFOUT

REFIN

0.1µF

10µF

Internal Reference Buffer Transient Response

The REFOUT pin of the LTC2311-16 draws charge (QCONV)

from the external bypass capacitors during each conver-

sion cycle. If the internal reference buffer is overdriven,

the external reference must provide all of this charge

with a DC current equivalent to IREFOUT = QCONV/tCYC.

Thus, the DC current draw of REFOUT depends

on the sampling rate and output code. In applications

where a burst of samples is taken after idling for long

periods, as shown in Figure 14 , IREFOUT quickly goes from

approximately ~75µA to a maximum of 700µA for REFOUT

= 5V at 5Msps. This step in DC current draw triggers a

transient response in the external reference that must be

considered since any deviation in the voltage at REFOUT

Figure 14. CNV Waveform Showing Burst Sampling

Figure 15. Transient Response of the LTC2311-16

CNV

231116 F14

IDLE

PERIOD

231116 F15

TIME (ns)

200

100

35000

30000

25000

20000

15000

10000

5000

–5000

DYNAMIC PERFORMANCE

Fast Fourier transform (FFT) techniques are used to test

the ADC’s frequency response, distortion and noise at the

rated throughput. By applying a low distortion sine wave

and analyzing the digital output using an FFT algorithm,

the ADC’s spectral content can be examined for frequen-

cies outside the fundamental. The LTC2311-16 provides

guaranteed tested limits for both AC distortion and noise

measurements.

Signal-to-Noise and Distortion Ratio (SINAD)

The signal-to-noise and distortion ratio (SINAD) is the

ratio between the RMS amplitude of the fundamental input

frequency and the RMS amplitude of all other frequency

components at the A/D output. The output is bandlimited

to frequencies from above DC and below half the sampling

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