19
LTC1594L/LTC1598L
15948lfb
specification in the Dynamic Accuracy table includes the
2nd through 5th harmonics. With a 1kHz input signal, the
LTC1594L/LTC1598L have typical THD of 78dB with
V
CC
= 2.7V.
Intermodulation Distortion
If the ADC input signal consists of more than one
spectral component, the ADC transfer function nonlin-
earity can produce intermodulation distortion (IMD)
in addition to THD. IMD is the change in one sinusoi-
dal input caused by the presence of another sinusoidal
input at a different frequency.
If two pure sine waves of frequencies f
a
and f
b
are applied
to the ADC input, nonlinearities in the ADC transfer
function can create distortion products at sum and differ-
ence frequencies of mf
a
± nf
b
, where m and n = 0, 1, 2, 3,
etc. For example, the 2nd order IMD terms include (f
a
+
f
b
) and (f
a
– f
b
) while 3rd order IMD terms include (2f
a
+
f
b
), (2f
a
– f
b
), (f
a
+ 2f
b
), and (f
a
– 2f
b
). If the two input sine
waves are equal in magnitudes, the value (in dB) of the
2nd order IMD products can be expressed by the follow-
ing formula:
APPLICATIONS INFORMATION
WUU
U
TYPICAL APPLICATIONS N
U
Microprocessor Interfaces
The LTC1594L/LTC1598L can interface directly (without
external hardware) to most popular microprocessors’
(MPU) synchronous serial formats including
MICROWIRE, SPI and QSPI. If an MPU without a dedi-
cated serial port is used, then three of the MPU’s parallel
port lines can be programmed to form the serial link to the
LTC1594L/LTC1598L. Included here is one serial interface
example.
Motorola SPI (MC68HC05)
The MC68HC05 has been chosen as an example of an MPU
with a dedicated serial port. This MPU transfers data MSB-
first and in 8-bit increments. The D
IN
word sent to the data
register starts the SPI process. With three
8-bit transfers the A/D result is read into the MPU. The
second 8-bit transfer clocks B11 through B7 of the A/D
conversion result into the processor. The third 8-bit trans-
fer clocks the remaining bits B6 through B0 into the MPU.
ANDing the second byte with 1F
HEX
clears the three most
significant bits and ANDing the third byte with FE
HEX
clears
the least significant bit. Shifting the data to the right by one
bit results in a right justified word.
IMD f f
mplitude f f
ab
ab
±
()
=
±
()
20log
a
amplitude at f
a
Peak Harmonic or Spurious Noise
The peak harmonic or spurious noise is the largest
spectral component excluding the input signal and DC.
This value is expressed in dBs relative to the RMS value
of a full-scale input signal.
Full-Power and Full-Linear Bandwidth
The full-power bandwidth is that input frequency at
which the amplitude of the reconstructed fundamental is
reduced by 3dB for a full-scale input.
The full-linear bandwidth is the input frequency at which
the effective bits rating of the ADC falls to 11 bits. Beyond
this frequency, distortion of the sampled input signal
increases. The LTC1594L/LTC1598L have been designed
to optimize input bandwidth, allowing the ADCs to
undersample input signals with frequencies above the
converters’ Nyquist Frequency.
