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LTC2385-18

238518f

For more information www.linear.com/LTC2385-18

APPLICATIONS INFORMATION

Figure 10. 32k Point FFT of the LTC2385-

18, fSMPL = 5Msps, fIN = 2kHz

Common Mode Output

The VCM pin is an output that provides one-half the voltage

present on the REFBUF pin. This voltage can be used to

set the common mode of a differential amplifier driving the

analog inputs. Bypass VCM to GND with a 0.1μF ceramic

capacitor. If VCM is not used it can be left floating, but the

parasitic capacitance on the pin needs to be under 10pF.

The VCMoutputhas1/fnoisewhichformostdrivercircuits

will be removed by the ADC common mode rejection ratio.

VCM is not recommended for single-ended to differential

circuits that pass the VCM noise to only one ADC input.

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 LTC2385-18 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

to frequencies from above DC and below half the sampling

frequency. Figure 10 shows that the LTC2385-18 achieves

a typical SINAD of 96.1dB at a 5MHz sampling rate with

a 2kHz 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 10 shows

that the LTC2385-18 achieves a typical SNR of 96.2dB at

a 5MHz sampling rate with a 2kHz input.

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 +…+ Vn 2

where V1 is the RMS amplitude of the fundamental

frequency and V2 through Vn are the amplitudes of the

second through nth harmonics. Figure 10 shows that the

LTC2385-18 achieves a typical THD of –117dB at a 5MHz

sampling rate with a 2kHz input.

POWER CONSIDERATIONS

The LTC2385-18 requires three power supplies: VDD (5V),

VDDL (2.5V), and OVDD (2.5V). Bypass VDD to GND with

a 0.1µF ceramic capacitor close to the pair of pins and a

SNR = 96.2dB

THD = –117dB

SINAD = 96.1dB

SFDR = 119dB

FREQUENCY (MHz)

0.5

1.5

2.5

–160

–140

–120

–100

–80

–60

–40

–20

238518 F10

Figure 9. Overdriving REFBUF Using the LTC6655-4.096

238518 F09

REFGND

REFBUF

REFIN

LTC2385-18

REFGND

REFBUF

10µF

VIN

LTC6655-4.096

GND

SHDN

VOUT_F

VOUT_S

0.1µF

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