Datasheet

LTC2389-16
27
238916f
ADC REFERENCE
A low noise, low temperature drift reference is critical
to achieving the full data sheet performance of the ADC.
The LTC2389-16 provides an excellent internal refer-
ence with a ±20ppm/°C (maximum) temperature coef-
ficient. If even better accuracy is required, an external
reference can be used. In both cases, the high speed, low
noise internal reference buffer is employed and cannot be
bypassed. The buffer contributes a signal-dependent noise
term to the converter with a typical standard deviation of:
(V
IN
+
−
V
IN
–
)
V
REF
• 16µV
RMS
,
which accounts for the increase in transition noise between
zero-scale and full-scale inputs. The reference voltage
applied to REFIN adds a similar signal-dependent noise
term, but its magnitude is limited by a 4kHz (typical)
lowpass filter in the internal buffer, making this term
negligible in most cases.
Internal Reference
To use the internal reference, simply tie the REFOUT and
REFIN pins together. This connects the 4.096V output of
the internal reference to the input of the internal reference
buffer. The output impedance of the internal reference
is approximately 2.3kΩ and the input impedance of the
internal reference buffer is about 74kΩ. It is recommended
REFIN be bypassed to REFSENSE with a 1μF, or larger,
capacitor to filter the output noise of the internal refer-
ence. Do not ground the REFSENSE pin when using the
internal reference.
External Reference
An external reference can be used with the LTC2389-16
when even higher performance is required. The
LTC6655 offers 0.025% (maximum) initial accuracy
and 2ppm/°C (maximum) temperature coefficient for
high precision applications. The LTC6655 is fully speci-
fied over the H-grade temperature range and complements
the extended temperature operation of the LTC2389-16
up to 125°
C. When using an external reference, connect
the reference output to the REFIN pin and connect the
REFOUT pin to ground. The REFSENSE pin should be
connected to the ground of the external reference.
applications inFormation
Figure 11. 32k Point FFT of LTC2389-16,
f
SMPL
= 2.5Msps, f
IN
= 2kHz
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 ana-
lyzing the digital output using an FFT algorithm, the ADC’s
spectral content can be examined for frequencies outside the
fundamental. The LTC2389-16 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
to frequencies from above DC and below half the sampling
frequency. Figure 11 shows that the LTC2389-16 achieves
a typical SINAD of 96.0dB (fully differential) at a 2.5MHz
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 11 shows
that the LTC2389-16 achieves a typical SNR of 96.0dB
(fully differential) at a 2.5MHz sampling rate with a
2kHz input.
FREQUENCY (kHz)
0
AMPLITUDE (dBFS)
–60
–40
–20
–80
–100
500250 750 1000 1250
–160
–180
–120
0
–140
238916 F11
SNR = 96.0dB
THD = –116dB
SINAD = 96.0dB
SFDR = 117dB