AD6620
INPUT DATA PORT
Thus for fixed-point ADCs, the exponents are typically static
and no input scaling is used in the AD6620.
The input data port accepts a clock (CLK), a 16-bit mantissa
IN[15:0], a 3-bit exponent EXP[2:0], and channel select Pin A/B.
These pins allow direct interfacing to both standard fixed-point
ADCs such as the AD9225 and AD6640, as well as to gain-
ranging ADCs such as the AD6600. These inputs are not 5 V
tolerant and the ADC I/O should be set to 3.3 V.
D11 (MSB)
IN15
AD6640
The input data port accepts data in one of three input modes:
Single Channel Real, Diversity Channel Real, or Single Channel
Complex. The input mode is selected by programming the Input
Mode Control Register located at internal address space 300h.
AD6620
D0 (LSB)
IN4
Single Channel Real mode is used when a single channel ADC
drives the input to the AD6620. Diversity Channel Real mode is
the two channel mode used primarily for diversity receiver appli-
cations. Single Channel Complex mode accepts complex data in
conjunction with the A/B input which identifies in-phase and
quadrature samples (primarily for cascaded 6620s).
IN3
IN2
IN1
IN0
EXP2
EXP1
EXP0
A/B
+3.3V
The input data port is sampled on the rising edge of CLK at a
maximum rate of 67 MSPS. The 16-bit mantissa, IN[15:0] is
interpreted as a twos complement integer. For most applications
with ADCs having fewer than 16 bits, the active bits should be
MSB justified and the unused LSBs should be tied low.
Figure 21. Typical Interconnection of the AD6640 Fixed
Point ADC and the AD6620
Scaling with Floating-Point ADCs
An example of the exponent control feature combines the AD6600
and the AD6620. The AD6600 is an 11-bit ADC with three bits
of gain ranging. In effect, the 11-bit ADC provides the mantissa,
and the three bits of relative signal strength indicator (RSSI) are
the exponent. Only five of the eight available steps are used by
the AD6600. See the AD6600 data sheet for additional details.
The 3-bit exponent, EXP[2:0] is interpreted as an unsigned
integer. The exponent can be modified by the 3-bit exponent
offset ExpOff (Control Register 0x305, Bits (7–5)) and an expo-
nent invert ExpInv (Control Register 0x305, Bit 4).
ExpOff sets the offset of the input exponent, EXP[2:0]. ExpInv
determines the direction of this offset. Equations below show
how the exponent is handled.
For gain-ranging ADCs such as the AD6600,
scaled _input = IN ×2– mod(7–Exp+ExpOff,8), ExpInv =1
scaled _input = IN ×2– mod(Exp+ExpOff,8), ExpInv = 0
scaled _input = IN ×2– mod(7–Exp+ExpOff,8), ExpInv =1
where: IN is the value of IN[15:0], Exp is the value of EXP[2:0],
and ExpOff is the value of ExpOff.
The RSSI output of the AD6600 numerically grows with increas-
ing signal strength of the analog input (RSSI = 5 for a large
signal, RSSI = 0 for a small signal). With the Exponent Offset
equal to zero and the Exponent Invert Bit equal to zero, the
AD6620 would consider the smallest signal at the parallel input
(EXP = 0) the largest and, as the signal and EXP word increase,
it shifts the data down internally (EXP = 5, will shift the 11-bit
data right by 5 bits internally before going into the CIC2). The
AD6620 regards the largest signal possible on the AD6600 as
the smallest signal. Thus the Exponent Invert Bit is used to make
the AD6620 exponent agree with the AD6600 RSSI. When it
is set high, it forces the AD6620 to shift the data up for growing
EXP instead of down. The exponent invert bit should always be
set high for use with the AD6600.
where: IN is the value of IN[15:0], Exp is the value of EXP[2:0],
and ExpOff is the value of ExpOff.
Input Scaling
In general there are two reasons for scaling digital data. The
first is to avoid “clipping” or, in the case of the AD6620 regis-
ter, “wrap-around” in subsequent stages. Wrap-around is not a
concern for the input data since the NCO is designed to accept
the largest possible input at the AD6620 data port.
The second use of scaling is to preserve maximum dynamic
range through the chip. As data flows from one stage to the next
it is important to keep the math functions performed in the
MSBs. This will keep the desired signal as far above the noise
floor as possible, thus maximizing signal-to-noise ratio.
Table I. AD6600 Transfer Function with AD6620 ExpInv = 1,
and No ExpOff
Scaling with Fixed-Point ADCs
For fixed-point ADCs, the AD6620 exponent inputs EXP[2:0]
are typically not used and should be tied low. The ADC outputs
are tied directly to the AD6620 Inputs, MSB-justified. The
exponent offset (ExpOff) and exponent invert (ExpInv) should
both be programmed to 0. Thus the input equation,
ADC Input
Level
AD6600
RSSI[2.0]
AD6620
Data
Signal
Reduction
Largest
101 (5)
100 (4)
011 (3)
010 (2)
001 (1)
000 (0)
Ϭ 4 (>> 2)
Ϭ 8 (>> 3)
Ϭ 16 (>> 4)
Ϭ 32 (>> 5)
Ϭ 64 (>> 6)
Ϭ 128 (>> 7)
–12 dB
–18 dB
–24 dB
–30 dB
–36 dB
–42 dB
scaled _input = IN ×2– mod(Exp+ExpOff,8), ExpInv = 0
where: IN is the value of IN[15:0], Exp is the value of EXP[0:2],
and ExpOff is the value of ExpOff, simplifies to,
Smallest
scaled _input = IN × 2– mod(0, 8)
(ExpInv = 1, ExpOff = 0)
REV. A
–15–
ADI [ ADI ]
