the LSB at the beginning of the conversion cycle
toprovideanoutputvoltagefromtheCDACthat
is equal to the input signal voltage (which is
divided by the input voltage divider network).
Thecomparatordetermineswhethertheaddition
ofeachsuccessively–weightedbitvoltagecauses
theCDACoutputvoltagesummationtobegreater
or less than the input voltage; if the sum is less,
the bit is left on; if more, the bit is turned off.
After testing all the bits, the SAR contains a 12–
bit binary code which accurately represents the
input signal to within ±1⁄2 LSB.
FEATURES…
The HS574A/SP674A feature standard bipolar
and unipolar input ranges of 10V and 20V. Input
ranges are controlled by a bipolar offset pin and
laser-trimmed for specified linearity, gain and
offset accuracy. Power requirements are +5V
and +12V to +15V with a maximum dissipation
of 150mW at the specified voltages. Conversion
times of 8µs, 10µs, 15µs and 25µs are available,
asareunitswith10,25or50ppm/°Ctemperature
coefficients for flexible matching to specific
application requirements.
Theinternalreferenceprovidesthevoltagerefer-
ence to the CDAC with excellent stability over
temperature and time. The reference is trimmed
to 10.00 Volts ±1% and can supply up to 2mA to
an external load in addition to that required to
drive the reference input resistor (1mA) and
offset resistor (1mA) when operating with ±15V
supplies. If the HS574A/SP674A is used with
±12V supplies, or if external current must be
supplied over the full temperature range, an
external buffer amplifier is recommended. Any
externalloadonthe HS574A/SP674Areference
must remain constant during conversion.
The HS574A/SP674A are available in six prod-
uct grades for each conversion time. The –J, –K
and –L models are specified over 0˚C to + 70˚C
commercial temperature range; the –S, –T and –
Umodelsarespecifiedoverthe–55˚Cto+125˚C
military temperature range. Processing in accor-
dance with MIL–STD–883C is also available.
The HS574A/SP674A are packaged in a 28–pin
CerDIP. Please consult the factory for other
packaging options.
CIRCUIT OPERATION…
The HS574A/SP674A are complete 12–bit ana-
log-to-digital converters with integral voltage
reference, comparator, successive–approxima-
tion register (SAR), sample–and–hold, clock,
output buffers and control circuitry. The high
level of integration of the HS574A/SP674A
means they require few external components.
The sample and hold is a default function by
virtue of the CDAC architecture. Therefore the
majority of the S/H specifications are included
within the A/D specifications.
Sample–and–Hold Function
Although there is no sample-and-hold circuit in
the classical sense, the sampling nature of the
capacitive DAC makes the HS574A/SP674A
appear to have a built in sample and hold. This
sample and hold action substantially increases
the usefulness of the HS574A/SP674Aover that
of similar competing devices.
WhenthecontrolsectionoftheHS574A/SP674A
initiates a conversion command, the clock is
enabled and the successive–approximation reg-
ister is reset to all zeros. Once the conversion
cycle begins, it can not be stopped or re–started
and data is not available from the output buffers.
Note that even though the user may use an
external sample and hold for very high fre-
quency inputs, the internal sample and hold still
provides a very useful isolation function. Once
theinternalsampleistakenbytheCDACcapaci-
tance, the input of the HS574A/SP674A is dis-
connected from the input. This prevents tran-
sients occurring during conversion from being
inflicted upon the attached buffer. All other 574/
674circuitswillcauseatransientloadcurrenton
TheSAR, timedbytheclock, sequencesthrough
the conversion cycle and returns an end–of–
convert flag to the control section of the ADC.
The clock is then disabled by the control section,
the output status goes low, and the control sec-
tion is enabled to allow the data to be read by
external command.
The internal HS574A/SP674A 12–bit CDAC is
sequenced by the SAR starting from the MSB to
7
SPECTRUM [ SPECTRUM MICROWAVE, INC. ]
