Datasheet

MAX153 1Msps, µP-Compatible,
8-Bit ADC with 1µA Power-Down
www.maximintegrated.com
Maxim Integrated │ 11
Conversion Rate
The maximum sampling rate (f
MAX
) for the MAX153 is
achievedintheWR-RDmode(t
RD
< t
INTL
) and is calcu-
latedasfollows:
MAX
WR RD RI P
MAX
MAX
1
f
t t tt
1
f
250ns 250ns 150ns 165ns
f 1.23MHz
=
+ ++
=
+++
=
where t
WR
= Write pulse width
t
RD
= Delay between WR and RD pulses
t
RI
= RD to INT delay
t
P
= Delay time between conversions
Signal-to-Noise Ratio and Effective
Number of Bits
Signal-to-noiseratio(SNR)istheratiooftheRMSampli-
tude of the fundamental input frequency to the RMS
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
noisecomponents.Forthisreason,theratioissometimes
referred to as signal-to-noise plus distortion.
The theoretical minimum A/D noise is caused by quanti-
zationerrorandresultsdirectlyfromtheADC’sresolution:
SNR=(6.02N+1.76)dB,whereNisthenumberofbits
of resolution. Therefore, a perfect 8-bit ADC can do no
betterthan50dB.
The FFTplot (Typical Operating Characteristics) shows
theresultofsamplingapure200kHzsinusoidata1MHz
rate.ThisFFTplotoftheoutputshowstheoutputlevelin
various spectral bands.
The effective resolution, or effective number of bits, the
ADC provides can be measured by transposing the
equation that converts resolution to SNR: N = (SNR -
1.76)/6.02.
Total Harmonic Distortion
Total harmonic distortion (THD) is the ratio of the RMS
sum of all harmonics of the input signal (in the frequency
band above DC and below one-half the sample rate) to
thefundamentalitself.Thisisexpressedas:
2 22 2
2 34 N
1
(V + V +V + ... +V )
THD 20log
V


=



where V
1
isthefundamentalRMSamplitude,andV
2
to
V
N
aretheamplitudesofthe2ndthroughNthharmonics.
Peak Harmonic or Spurious Noise
Peakharmonicorspuriousnoiseistheratioofthefunda-
mentalRMSamplitudetotheamplitudeofthenextlargest
spectral component (in the frequency band above DC and
belowone-halfthesamplerate).Usuallythispeakoccurs
at some harmonic of the input frequency, but if the ADC is
exceptionallylinear,itmayoccuronlyatarandompeak
in the ADC’s noise floor.
Intermodulation Distortion
AnFFTplotofintermodulationdistortion(IMD)isgener-
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.
Figure 8. Equivalent Input Circuit Figure 9. RC Network Equivalent Input Model
V
IN
V
IN
R
ON
R
IN
1
C
MAX153
V
IN
V
IN
2kΩR
1
12pF
MAX153
10pF