Datasheet

AD9640
Rev. B | Page 32 of 52
ADC OVERRANGE AND GAIN CONTROL
In receiver applications, it is desirable to have a mechanism to
reliably determine when the converter is about to be clipped.
The standard overflow indicator provides after-the-fact infor-
mation on the state of the analog input that is of limited usefulness.
Therefore, it is helpful to have a programmable threshold below
full scale that allows time to reduce the gain before the clip
actually occurs. In addition, because input signals can have
significant slew rates, latency of this function is of major concern.
Highly pipelined converters can have significant latency. A good
compromise is to use the output bits from the first stage of the
ADC for this function. Latency for these output bits is very low,
and overall resolution is not highly significant. Peak input signals
are typically between full scale and 6 dB to 10 dB below full
scale. A 3-bit or 4-bit output provides adequate range and
resolution for this function.
Using the SPI port, the user can provide a threshold above which
an overrange output is active. As long as the signal is below that
threshold, the output should remain low. The fast detect outputs
can also be programmed via the SPI port so that one of the pins
functions as a traditional overrange pin for customers who
currently use this feature. In this mode, all 14 bits of the converter
are examined in the traditional manner, and the output is high for
the condition normally defined as overflow. In either mode, the
magnitude of the data is considered in the calculation of the
condition (but the sign of the data is not considered). The threshold
detection responds identically to positive and negative signals
outside the desired range (magnitude).
FAST DETECT OVERVIEW
The AD9640 contains circuitry to facilitate fast overrange detec-
tion, allowing very flexible external gain control implementations.
Each ADC has four fast detect (FD) output pins that are used
to output information about the current state of the ADC input
level. The function of these pins is programmable via the fast detect
mode select bits and the fast detect enable bit in Register 0x104,
allowing range information to be output from several points in
the internal datapath. These output pins can also be set up to
indicate the presence of overrange or underrange conditions,
according to programmable threshold levels. Table 17 shows the
six configurations available for the fast detect pins.
Table 17. Fast Detect Mode Select Bits Settings
Fast Detect
Mode Select Bits
(Register 0x104[3:1])
Information Presented on
Fast Detect (FD) Pins of Each ADC
1, 2
FD3 FD2 FD1 FD0
000
ADC fast magnitude
(see Table 18)
001
ADC fast magnitude
(see Table 19)
OR
010
ADC fast magnitude
(see Table 20)
OR F_LT
011
ADC fast magnitude
(see Table 20)
C_UT F_LT
100 OR C_UT F_UT F_LT
101 OR F_UT IG DG
1
The fast detect pins are FD0A/FD0B to FD3A/FD3B for the CMOS mode
configuration and FD0+/FD0− to FD3+/FD3− for the LVDS mode configuration.
2
See the ADC Overrange (OR) and Gain Switching sections for more
information about OR, C_UT, F_UT, F_LT, IG, and DG.
ADC FAST MAGNITUDE
When the fast detect output pins are configured to output the ADC
fast magnitude (that is, when the fast detect mode select bits are
set to 0b000), the information presented is the ADC level from
an early converter stage with a latency of only two clock cycles
(when in CMOS output mode). Using the fast detect output pins
in this configuration provides the earliest possible level indication
information. Because this information is provided early in the
datapath, there is significant uncertainty in the level indicated.
The nominal levels, along with the uncertainty indicated by the
ADC fast magnitude, are shown in Table 18.
Table 18. ADC Fast Magnitude Nominal Levels with Fast Detect
Mode Select Bits = 000
ADC Fast
Magnitude on
FD[3:0] Pins
Nominal Input
Magnitude
Below FS (dB)
Nominal Input
Magnitude
Uncertainty (dB)
0000 <−24 Minimum to −18.07
0001 −24 to −14.5 −30.14 to −12.04
0010 −14.5 to −10 −18.07 to −8.52
0011 −10 to −7 −12.04 to −6.02
0100 −7 to −5 −8.52 to −4.08
0101 −5 to −3.25 −6.02 to −2.5
0110 −3.25 to −1.8 −4.08 to −1.16
0111 −1.8 to −0.56 −2.5 to FS
1000 −0.56 to 0 −1.16 to 0