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LTC2379-18

237918p

Figure 11. 32k Point FFT of the LTC2379-18

FREQUENCY (kHz)

100 200 300 400

800

700

600

500

–180

–60

–40

–20

–80

–100

–120

–140

–160

237918 F11

SNR = 101.2dB

THD = –120dB

SINAD = 101.1dB

SFDR = 121dB

APPLICATIONS INFORMATION

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

code. In applications where the transient response of the

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

ence is also recommended.

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 LTC2379-18 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 11 shows that the LTC2379-18 achieves

a typical SINAD of 100.5dB 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

the RMS amplitude of all other frequency components

except the first five harmonics and DC. Figure 11 shows

that the LTC2379-18 achieves a typical SNR of 101dB at

a 1.6MHz sampling rate with a 2kHz input.

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 LTC2379-18 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 LTC2379-18 to communicate with any digital

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

3.3V systems.

Power Supply Sequencing

The LTC2379-18 does not have any specific power supply

sequencing requirements. Care should be taken to adhere

to the maximum voltage relationships described in the

Absolute Maximum Ratings section. The LTC2379-18

has a power-on-reset (POR) circuit that will reset the

LTC2379-18 at initial power-up or whenever the power

supply voltage drops below 1V. Once the supply voltage

reenters the nominal supply voltage range, the POR will

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