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LTC2345-16

234516f

For more information www.linear.com/LTC2345-16

conversions following an idle period. Figure 13 compares

the burst conversion response of the LTC2345-16 with an

input near full scale for two reference configurations. The

first configuration employs the internal reference buffer

with REFIN externally overdriven by an LTC6655-2.048,

while the second configuration disables the internal ref-

erence buffer and overdrives REFBUF with an external

LTC6655-4.096. In both cases REFBUF is bypassed to

GND with a 47µF ceramic capacitor.

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 LTC2345-16 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 band-limited

applicaTions inForMaTion

Figure 13. Burst Conversion Response of the LTC2345-16,

fSMPL = 200ksps

to frequencies below half the sampling frequency, exclud-

ing DC. Figure 14 shows that the LTC2345-16 achieves a

typical SINAD of 91.1dB in the ±4.096V range at a 200kHz

sampling rate with a fully differential 2kHz input signal.

Figure 14. 32k Point FFT fSMPL = 200ksps, fIN = 2kHz

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 14 shows

that the LTC2345-16 achieves a typical SNR of 91.1dB in

the ±4.096V range at a 200kHz sampling rate with a fully

differential 2kHz input signal.

Total Harmonic Distortion (THD)

Totalharmonicdistortion(THD)istheratiooftheRMSsum

ofallharmonicsoftheinputsignaltothefundamentalitself.

The out-of-band harmonics alias into the frequency band

between DC and half the sampling frequency (fSMPL/2).

THD is expressed as:

THD

= 20log

V22 + V32 + V42...VN2

where V1 is the RMS amplitude of the fundamental fre-

quencyandV2throughVNaretheamplitudesofthesecond

through Nth harmonics, respectively. Figure 14 shows

±4.096V RANGE

FULLY DIFFERENTIAL DRIVE (IN– = –IN+)

SNR = 91.1dB

THD = –111dB

SINAD = 91.1dB

SFDR = 112dB

FREQUENCY (KHz)

100

–180

–160

–140

–120

–100

–80

–60

–40

–20

234516 F14

INTERNAL REFERENCE BUFFER

EXTERNAL REFERENCE ON REFBUF

±4.096V SOFTSPAN

IN+ = 4V

IN– = 0V

TIME (µs)

100

200

300

400

500

–2

–1

234516 F13

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