Rev. A
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Page 61 of 80
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July 2011
ADSP-BF504/ADSP-BF504F/ADSP-BF506F
determining how much that signal is attenuated in the
selected channel with a 50 kHz signal (0 V to VREF). The result
obtained is the worst-case across all 12 channels for the ADC.
Intermodulation Distortion (IMD)
With inputs consisting of sine waves at two frequencies, fa
and fb, any active device with non-linearities create distortion
products at sum, and difference frequencies of mfa ± nfb
where m, n = 0, 1, 2, 3, and so on. Intermodulation distortion
terms are those for which neither m nor n are equal to zero.
For example, the second-order terms include (fa + fb) and
(fa fb), while the third-order terms include (2fa + fb),
(2fa fb), (fa + 2fb), and (fa 2fb).
The ADC is tested using the CCIF standard where two input
frequencies near the top end of the input bandwidth are used.
In this case, the second-order terms are usually distanced in
frequency from the original sine waves, while the third-order
terms are usually at a frequency close to the input frequencies.
As a result, the second-order and third-order terms are speci-
fied separately. The calculation of the inter-modulation
distortion is as per the THD specification, where it is the ratio
of the rms sum of the individual distortion products to the
rms amplitude of the sum of the fundamentals expressed in
dBs.
Common-Mode Rejection Ratio (CMRR)
CMRR is defined as the ratio of the power in the ADC output
at full-scale frequency, f, to the power of a 100 mV p-p sine
wave applied to the common-mode voltage of VIN+ and VIN of
frequency fS as:
CMRR (dB) = 10 log(Pf/PfS)
where:
Pf is the power at frequency f in the ADC output.
PfS is the power at frequency fS in the ADC output.
Power Supply Rejection Ratio (PSRR)
Variations in power supply affect the full-scale transition but
not the converter’s linearity. PSRR is the maximum change in
the full-scale transition point due to a change in power supply
Thermal Hysteresis
Thermal hysteresis is defined as the absolute maximum
change of reference output voltage (VREF) after the device is
cycled through temperature from either:
T_HYS+ = +25°C to TMAX to +25°C
or
T_HYS = +25°C to TMIN to +25°C
It is expressed in ppm by:
where:
VREF (25°C) is VREF at 25°C.
VREF (T_HYS) is the maximum change of VREF at
T_HYS+ or T_HYS.
ADC—THEORY OF OPERATION
The following sections describe the ADC theory of operation.
Circuit Information
The ADC is a fast, micropower, dual, 12-bit, single-supply,
ADC that operates from a 2.7 V to a 5.25 V supply. When oper-
ated from a 5 V supply, the ADC is capable of throughput rates
of up to 2 MSPS when provided with a 32 MHz clock, and a
throughput rate of up to 1.5 MSPS at 3 V.
The ADC contains two on-chip, differential track-and-hold
amplifiers, two successive approximation ADCs, and a serial
interface with two separate data output pins.
The serial clock input accesses data from the part but also pro-
vides the clock source for each successive approximation ADC.
The analog input range for the part can be selected to be a 0 V to
VREF input or a 2 × VREF input, configured with either single-
ended or differential analog inputs. The ADC has an on-chip
2.5 V reference that can be overdriven when an external refer-
ence is preferred. If the internal reference is to be used elsewhere
in a system, then the output needs to buffered first.
The ADC also features power-down options to allow power sav-
ing between conversions. The power-down feature is
implemented via the standard serial interface, as described in
Converter Operation
The ADC has two successive approximation ADCs, each based
schematics of one of these ADCs in acquisition and conversion
phase, respectively. The ADC is comprised of control logic, a
Phase) (the acquisition phase), SW3 is closed, SW1 and SW2 are
in Position A, the comparator is held in a balanced condition,
and the sampling capacitor arrays acquire the differential signal
on the input.
6
10
)
C
25
(
)
_
(
)
C
25
(
)
(
×
°
°
=
REF
HYS
V
HYS
T
V
ppm
V
Figure 62. ADC Acquisition Phase
CAPACITIVE
DAC
CAPACITIVE
DAC
CONTROL
LOGIC
COMPARATOR
SW3
SW1
A
B
SW2
CS
VIN+
VIN–
VREF