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LTC1598LIG

4- and 8-Channel, 3V Micropower Sampling 12-Bit Serial I/O A/D Converters

Linear Technology 

LTC1594L/LTC1598L

APPLICATIONS INFORMATION

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

VCC = 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 fa and fb are applied

to the ADC input, nonlinearities in the ADC transfer

function can create distortion products at sum and differ-

ence frequencies of mfa ± nfb, where m and n = 0, 1, 2, 3,

etc. For example, the 2nd order IMD terms include (fa +

fb) and (fa – fb) while 3rd order IMD terms include (2fa +

fb), (2fa – fb), (fa + 2fb), and (fa – 2fb). 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:

( ) ( ) IMD fa ± fb

amplitude

= 20log 

fa

± fb

 amplitude at fa







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.

TYPICAL APPLICATIONS N

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 DIN 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 1FHEX clears the three most

significant bits and ANDing the third byte with FEHEX clears

the least significant bit. Shifting the data to the right by one

bit results in a right justified word.

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