AD6620
The Exponent Offset is used to shift the data right. For example,
Table I shows that with no ExpOff shift, 12 dB of range is
lost when the ADC input is at the largest level. This is undesired
because it lowers the Dynamic Range and SNR of the system
by reducing the signal of interest relative to the quantization
noise floor.
clock is typically used to clock the AD6620. Applications that
require a faster signal processing clock than the ADC sample
clock, may employ fractional rate input timing as shown in the
following sections. The input timing requirements vary according
to the mode of operation. Fractional rate input timing creates a
longer “don’t care” time for the input data so that slower ADCs
need only meet the setup-and-hold conditions for their data
with respect to their own sample clock cycle, rather than the
faster signal processing clock. The ADC sample clock may be
any integer fraction of CLK up to and including 1, as long as
the clock and data rate are less than or equal to 67 MSPS.
To avoid this automatic attenuation of the full-scale ADC sig-
nal, the Exponent Offset is used to move the largest signal (RSSI =
5) up to the point where there is no downshift. In other words,
once the Exponent Invert bit has been set, the Exponent Offset
should be adjusted so that mod(7–5 + ExpOff,8) = 0. This is
the case when Exponent Offset is set to 6 since mod(8, 8) = 0.
Table II illustrates the use of ExpInv and ExpOff when used
with the AD6600 ADC.
Single Channel Real Mode
In the Single Channel Real mode the A/B input pin functions as
an active high input enable. If the A/D sample clock is fast enough
to perform the necessary filter functions, full rate input timing
can be used and A/B should be tied high as shown in Figure 23.
Table II. AD6600 Transfer Function with AD6620 ExpInv = 1,
and ExpOff = 6
ADC Input
Level
AD6600
RSSI[2.0]
AD6620
Data
Signal
Reduction
CLK
tSI
tHI
Largest
101 (5)
100 (4)
011 (3)
010 (2)
001 (1)
000 (0)
Ϭ 1 (>> 0)
Ϭ 2 (>> 1)
Ϭ 4 (>> 2)
Ϭ 8 (>> 3)
Ϭ 16 (>> 4)
Ϭ 32 (>> 5)
–0 dB
–6 dB
–12 dB
–18 dB
–24 dB
–30 dB
IN[15:0]
N
N+1
N+2
N+3
N+4
EXP[2:0]
A/B
Smallest
Figure 23. Full Rate Input Timing, Single Channel
Real Mode
(ExpInv = 1, ExpOff = 6)
This flexibility in handling the exponent allows the AD6620 to
interface with other gain ranging ADCs besides the AD6600.
The Exponent Offset can be adjusted to allow up to seven
RSSI(EXP) ranges to be used as opposed to the AD6600s five.
It also allows the AD6620 to be tailored in a system that employs
the AD6600, but does not utilize all of its signal range. For
example, if only the first four RSSI ranges are expected to occur
then the Exponent Offset could be adjusted to five, which would
then make RSSI = 4 correspond to the 0 dB point of the AD6620.
When a faster processing clock is used to achieve better filter
performance, the A/D data must be synchronized with the faster
AD6620 CLK signal. This is achieved by having the ADC clock
rate an integer fraction of the AD6620 clock rate. AD6620 input
data is sampled at the slower ADC clock rate. In the Single
Channel Real Mode this is achieved by dynamically controlling
the A/B input and bringing it high before each rising CLK edge
that data is to be sampled on. A/B must be returned low before
the next high speed clock pulse and the duty cycle of the A/B
signal will therefore be equal to the data-to-clock ratio.
D10 (MSB)
IN15
CLK
tHI
tSI
AD6600
AD6620
IN[15:0]
N
N+1
EXP[2:0]
D0 (LSB)
IN4
IN3
A/B
IN2
IN1
IN0
EXP2
EXP1
EXP0
Figure 24. Fractional Rate Input Timing (4× CLK), Single
RSS12
RSS11
RSS10
Channel Real Mode
Diversity Channel Real Mode
A/B
A/B OUT
In the Diversity Channel Real mode the A/B pin serves not only
as an input enable but also to determine which channel is being
sampled on a given CLK edge. A high on the A/B pin marks
channel A data and a low on A/B marks channel B data. The
AD6620 only accepts the first sample after an A/B transition.
All subsequent samples are disregarded until A/B changes again.
Figure 22. Typical Interconnection of the AD6600 Gain-
Ranging ADC and the AD6620 in a Diversity Application
Input Timing
The CLK signal is used to sample the input port and clock the
synchronous signal processing stages that follow. The CLK signal
can operate up to 67 MHz and have a duty cycle of 45% to
55%. In applications using high speed ADCs, the ADC sample
When full rate input timing is employed in the Diversity Chan-
nel Real mode, A/B must toggle on every rising edge of CLK for
new data to be clocked into the AD6620.
REV. A
–16–
ADI [ ADI ]
