LTC1594 Linear Technology, LTC1594 Datasheet - Page 19

• Manufacturer Part Number: LTC1594
• Description: 4- and 8-Channel/ Micropower Sampling 12-Bit Serial I/O A/D Converters
• Manufacturer: Linear Technology

APPLICATIONS
specification in the Dynamic Accuracy table includes the
2nd through 5th harmonics. With a 7kHz input signal, the
LTC1594/LTC1598 have typical THD of 80dB with V
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
to the ADC input, nonlinearities in the ADC transfer
function can create distortion products at sum and
difference frequencies of mf
1, 2, 3, etc. For example, the 2nd order IMD terms include
(f
(2f
sine waves are equal in magnitudes, the value (in dB) of
the 2nd order IMD products can be expressed by the
following formula:
TYPICAL APPLICATIONS
Microprocessor Interfaces
The LTC1594/LTC1598 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
LTC1594/LTC1598. Included here is one serial interface
example.
a
a
IMD f
+ f
+ f
b
b
) and (f
), (2f
a
f
a
b
– f
a
– f
b
), (f
20log
b
U
) while 3rd order IMD terms include
a
+ 2f
a
INFORMATION
b
mplitude f
U
amplitude at f
), and (f
a
nf
U
a
b
W
– 2f
, where m and n = 0,
a
a
b
and f
N
). If the two input
f
a
b
b
are applied
U
CC
= 5V.
For input frequencies of 5kHz and 6kHz, the IMD of the
LTC1594/LTC1598 is 73dB with a 5V supply.
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 re-
duced 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 LTC1594/LTC1598 have been designed to
optimize input bandwidth, allowing the ADCs to
undersample input signals with frequencies above the
converters’ Nyquist Frequency.
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
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
significant bits and ANDing the third byte with FE
the least significant bit. Shifting the data to the right by one
bit results in a right justified word.
LTC1594/LTC1598
HEX
IN
clears the three most
word sent to the data
HEX
19
clears