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LTC2369-18

236918fa

Table 1. SNR, THD vs RIN for ±10V Input Signal

RIN

(Ω)

SNR

(dB)

THD

(dB)

499

402

96.1

–97.3

10k

2.49k

10k

–92

100k

24.9k

100k

20k

93.8

–93.5

APPLICATIONS INFORMATION

ADC REFERENCE

The LTC2369-18 requires an external reference to define

its input range. A low noise, low temperature drift refer-

ence is critical to achieving the full datasheet performance

of the ADC. Linear Technology offers a portfolio of high

performance references designed to meet the needs of

many applications. With its small size, low power and

high accuracy, the LTC6655-5 is particularly well suited for

use with the LTC2369-18. The LTC6655-5 offers 0.025%

(max) initial accuracy and 2ppm/°C (max) temperature

coefficient for high precision applications. The LTC6655-5

is fully specified over the H-grade temperature range and

complements the extended temperature operation of the

LTC2369-18 up to 125°C. We recommend bypassing the

LTC6655-5 with a 47μF ceramic capacitor (X5R, 0805 size)

close to the REF pin.

The REF pin of the LTC2369-18 draws charge (QCONV) from

the 47μF bypass capacitor during each conversion cycle.

The reference replenishes this charge with a DC current,

IREF = QCONV/tCYC. The DC current draw of the REF pin,

IREF, depends on the sampling rate and output code. If

the LTC2369-18 is used to continuously sample a signal

at a constant rate, the LTC6655-5 will keep the deviation

of the reference voltage over the entire code span to less

than 0.5LSBs.

When idling, the REF pin on the LTC2369-18 draws only

a small leakage current (< 1μA). In applications where a

burst of samples is taken after idling for long periods as

shown in Figure 6, IREF quickly goes from approximately

0μA to a maximum of 1.1mA at 1.6Msps. This step in DC

current draw triggers a transient response in the reference

that must be considered since any deviation in the refer-

ence output voltage will affect the accuracy of the output

Figure 6. CNV Waveform Showing Burst Sampling

Figure 5a. LT6202 Converting a ±10V Bipolar Signal

to a 0V to 5V Input Signal

Figure 5b. 32k Point FFT Plot with fIN = 2kHz

for Circuit Shown in Figure 5a

–

LT6202

402Ω

499Ω

RIN

499Ω

10μF

10V

–10V

200pF

VCM = VREF/2

236918 F05a

FREQUENCY (kHz)

–80

–60

–40

800

236918 F05b

–100

–120

–160

200

400

600

100

300

500

700

–140

–20

SNR = 96.1dB

THD = –97.3dB

SINAD = 92.7dB

SFDR = 97.5dB

CNV

IDLE

PERIOD

IDLE

PERIOD

236918 F06

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