LTC1992 datasheet(13/24 Pages) LINER

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LTC2369-18

236918fa

APPLICATIONS INFORMATION

code. In applications where the transient response of the

reference is important, the fast settling LTC6655-5 refer-

ence is also recommended.

In applications where power management is critical and

the external reference may be powered down, it is rec-

ommended that REF is kept greater than 2V in order to

guarantee a maximum shutdown current of 140μA. In such

applications, a Schottky diode can be placed between REF

and VDD, as shown in Figure 7.

the RMS amplitude of all other frequency components

except the first five harmonics and DC. Figure 8 shows

that the LTC2369-18 achieves a typical SNR of 96.5dB at

a 1.6MHz sampling rate with a 2kHz input.

FREQUENCY (kHz)

100 200 300 400

800

700

600

500

–180

–60

–40

–20

–80

–100

–120

–140

–160

236918 F08

SNR = 96.5dB

THD = –127dB

SINAD = 96.5dB

SFDR = 134dB

Figure 8. 32k Point FFT with fIN = 2kHz of the LTC2369-18

Figure 7. A Schottky Diode Between REF and VDD Maintains

REF > 2V for Applications Where the Reference May Be

Powered Down

REF

236918 F07

LTC2369-18

VDD

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 LTC2369-18 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 8 shows that the LTC2369-18 achieves

a typical SINAD of 96.5dB at a 1.6MHz 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

Total Harmonic Distortion (THD)

Total Harmonic Distortion (THD) is the ratio of the RMS sum

of all harmonics of the input signal to the fundamental itself.

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

2 + V32 + V42 +…+ V

where V1 is the RMS amplitude of the fundamental fre-

quency and V2 through VN aretheamplitudesofthesecond

through Nth harmonics.

POWER CONSIDERATIONS

The LTC2369-18 provides two power supply pins: the

2.5V power supply (VDD), and the digital input/output

interface power supply (OVDD). The flexible OVDD supply

allows the LTC2369-18 to communicate with any digital

logic operating between 1.8V and 5V, including 2.5V and

3.3V systems.

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