Minimum cycle time is accomplished by using DOUT’s

rising edge as the EOC signal. Clock out the data with

13 clock cycles at full speed. Raise

CS after the conver-

sion’s LSB has been read. After the specified minimum

next conversion.

Output Coding and Transfer Function

The data output from the MAX187/MAX189 is binary,

and Figure 10 depicts the nominal transfer function.

Code transitions occur halfway between successive

1 LSB = 1.00mV or 4.096V/4096.

_____________Dynamic Performance

High-speed sampling capability and a 75ksps through-

put make the MAX187/MAX189 ideal for wideband sig-

nal processing. To support these and other related

applications, Fast Fourier Transform (FFT) test tech-

niques are used to guarantee the ADC’s dynamic fre-

quency response, distortion, and noise at the rated

throughput. Specifically, this involves applying a low-

distortion sine wave to the ADC input and recording the

digital conversion results for a specified time. The data

is then analyzed using an FFT algorithm that deter-

mines its spectral content. Conversion errors are then

seen as spectral elements outside of the fundamental

input frequency. ADCs have traditionally been evaluat-

ed by specifications such as Zero and Full-Scale Error,

Integral Nonlinearity (INL), and Differential Nonlinearity

(DNL). Such parameters are widely accepted for speci-

fying performance with DC and slowly varying signals,

but are less useful in signal-processing applications,

where the ADC’s impact on the system transfer function

is the main concern. The significance of various DC

errors does not translate well to the dynamic case, so

different tests are required.

Signal-to-Noise Ratio and

Effective Number of Bits

Signal-to-noise plus distortion (SINAD) is the ratio of the

fundamental input frequency’s RMS amplitude to the

RMS amplitude of all other ADC output signals. The

input bandwidth is limited to frequencies above DC and

below one-half the ADC sample (conversion) rate.

The theoretical minimum ADC noise is caused by quan-

tization error and is a direct result of the ADC’s resolu-

tion: SINAD = (6.02N + 1.76)dB, where N is the number

of bits of resolution. An ideal 12-bit ADC can, therefore,

do no better than 74dB. An FFT plot of the output

shows the output level in various spectral bands. Figure

11 shows the result of sampling a pure 10kHz sine

wave at a 75ksps rate with the MAX187/MAX189.

+5V, Low-Power, 12-Bit Serial ADCs

______________________________________________________________________________________

13

11

…111

11

…110

11

…101

00

…011

00

…010

00

…001

00

…000

01

2

FS

OUTPUT CODE

FS - 3/2LSB

INPUT VOLTAGE (LSBs)

FS = +4.096V

1LSB =

FS

4096

FULL-SCALE

TRANSITION

3

20

0

-20

-40

-60

-80

-100

-120

-140

0

18.75

37.5

FREQUENCY (kHz)

Figure 11. MAX187/MAX189 FFT plot

Figure 10. MAX187/MAX189 Unipolar Transfer Function,

4.096V = Full Scale