MAX153
1Msps, µP-Compatible,
8-Bit ADC with 1µA Power-Down
MAX153
MAX153
R
2kΩ
12pF
1
V
IN
R
IN
R
ON
V
IN
1
V
IN
V
IN
10pF
C
Figure 8. Equivalent Input Circuit
Figure 9. RC Network Equivalent Input Model
The effective resolution, or effective number of bits, the
ADC provides can be measured by transposing the
equationꢀ thatꢀ convertsꢀ resolutionꢀ toꢀ SNR:ꢀ Nꢀ =ꢀ (SNRꢀ -ꢀ
1.76)/6.02.
Conversion Rate
The maximum sampling rate (f
achievedꢀinꢀtheꢀWR-RDꢀmodeꢀ(t
latedꢀasꢀfollows:
) for the MAX153 is
MAX
RD
< t ) and is calcu-
INTL
Total Harmonic Distortion
1
f
=
=
MAX
Totalꢀ harmonicꢀ distortionꢀ (THD)ꢀ isꢀ theꢀ ratioꢀ ofꢀ theꢀ RMSꢀ
sum of all harmonics of the input signal (in the frequency
band above DC and below one-half the sample rate) to
theꢀfundamentalꢀitself.ꢀThisꢀisꢀexpressedꢀas:
t
+ t
+ t + t
WR
RD
RI
P
1
f
f
MAX
MAX
250ns + 250ns +150ns +165ns
= 1.23MHz
2
2
2
2
(V
+ V
+V
+ ... +V
)
2
3
4
N
where t
= Write pulse width
THD = 20log
WR
V
1
t
t
= Delay between WR and RD pulses
RD
= RD to INT delay
RI
where V ꢀisꢀtheꢀfundamentalꢀRMSꢀamplitude,ꢀandꢀV to
1
2
t = Delay time between conversions
P
V ꢀareꢀtheꢀamplitudesꢀofꢀtheꢀ2ndꢀthroughꢀNthꢀharmonics.
N
Signal-to-Noise Ratio and Effective
Number of Bits
Peak Harmonic or Spurious Noise
Peakꢀharmonicꢀorꢀspuriousꢀnoiseꢀisꢀtheꢀratioꢀofꢀtheꢀfunda-
mentalꢀRMSꢀamplitudeꢀtoꢀtheꢀamplitudeꢀofꢀtheꢀnextꢀlargestꢀ
spectral component (in the frequency band above DC and
belowꢀone-halfꢀtheꢀsampleꢀrate).ꢀUsuallyꢀthisꢀpeakꢀoccursꢀ
at some harmonic of the input frequency, but if the ADC is
exceptionallyꢀlinear,ꢀitꢀmayꢀoccurꢀonlyꢀatꢀaꢀrandomꢀpeakꢀ
in the ADC’s noise floor.
Signal-to-noiseꢀratioꢀ(SNR)ꢀisꢀtheꢀratioꢀofꢀtheꢀRMSꢀampli-
tudeꢀ ofꢀ theꢀ fundamentalꢀ inputꢀ frequencyꢀ toꢀ theꢀ RMSꢀ
amplitude of all other analog-to-digital output values. The
output band is limited to one-half the A/D sample (conver-
sion) rate. This ratio usually includes distortion as well as
noiseꢀcomponents.ꢀForꢀthisꢀreason,ꢀtheꢀratioꢀisꢀsometimesꢀ
referred to as signal-to-noise plus distortion.
Intermodulation Distortion
The theoretical minimum A/D noise is caused by quanti-
zationꢀerrorꢀandꢀresultsꢀdirectlyꢀfromꢀtheꢀADC’sꢀresolution:ꢀ
SNRꢀ=ꢀ(6.02Nꢀ+ꢀ1.76)dB,ꢀwhereꢀNꢀisꢀtheꢀnumberꢀofꢀbitsꢀ
of resolution. Therefore, a perfect 8-bit ADC can do no
betterꢀthanꢀ50dB.
AnꢀFFTꢀplotꢀofꢀintermodulationꢀdistortionꢀ(IMD)ꢀisꢀgener-
ated by sampling an analog input applied to the ADC. This
input consists of very low distortion sine waves at two
frequencies. A 2048 point plot for IMD of the MAX153 is
shown in the Typical Operating Characteristics.
Theꢀ FFTꢀ plotꢀ (Typical Operating Characteristics) shows
theꢀresultꢀofꢀsamplingꢀaꢀpureꢀ200kHzꢀsinusoidꢀatꢀaꢀ1MHzꢀ
rate.ꢀThisꢀFFTꢀplotꢀofꢀtheꢀoutputꢀshowsꢀtheꢀoutputꢀlevelꢀinꢀ
various spectral bands.
Maxim Integrated
│ 11
www.maximintegrated.com
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
