LTC2602 datasheet(23/32 Pages) LINER

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LTC2320-12

Rev B

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APPLICATIONS INFORMATION

signal swing and corresponding SNR, the LTC6655-5 is

recommended when overdriving REFOUT. The LTC6655-5

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) close to each of the REFOUT1,2,3,4

pins. If the REF pin voltage is used as a REFOUT refer-

ence when REFBUFEN is connected to GND, it should be

buffered externally.

Internal Reference Buffer Transient Response

The REFOUT1,2,3,4 pins of the LTC2320-12 draw charge

(QCONV) from the external bypass capacitors during each

conversion cycle. If the internal reference buffer is over-

driven,theexternalreferencemustprovideallofthischarge

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

Thus, the DC current draw of IREFOUT1,2,3,4 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 , IREFBUF quickly goes from

approximately ~75µA to a maximum of 500µA for REFOUT

= 5V at 1.5Msps. 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

will affect the accuracy of the output code. If an external

reference is used to overdrive REFOUT1,2,3,4, the fast

settling LTC6655 reference is recommended.

CNV

232012 F14

IDLE

PERIOD

Figure 14. CNV Waveform Showing Burst Sampling

Figure 15. Transient Response of the LTC2320-12

4.096V RANGE

IN+ = 1.5MHz SQUARE WAVE

IN– = 0V

SETTLING TIME (ns)

–20 –10 0 10 20 30 40 50 60 70 80 90

–1024

1024

2048

3072

4096

232012 F15

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 LTC2320-12 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

frequency.Figure 16 showsthattheLTC2320-12achieves

a typical SINAD of 77dB at a 1.5MHz sampling rate with

a 500kHz input.

Signal-to-Noise Ratio (SNR)

The signal-to-noise ratio (SNR) is the ratio between the

RMS amplitude of the fundamental input frequency and

the RMS amplitude of all other frequency components

except the first five harmonics and DC. Figure 16 shows

that the LTC2320-12 achieves a typical SNR of 77dB at a

1.5MHz sampling rate with a 500kHz input.

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