16.3.1 Input Control and Adjust
There are several features and configurations for the input of
the ADC10D1000/1500 so that it may be used in many dif-
ferent applications. This section covers AC/DC-coupled
Mode, input full-scale range adjust, input offset adjust, DES/
Non-DES Mode, sampling clock phase adjust, an LC filter on
the sampling clock, and VCMO Adjust.
16.3.1.1 AC/DC-coupled Mode
The analog inputs may be AC or DC-coupled. See Sec-
tion 16.2.1.10 AC/DC-Coupled Mode Pin (VCMO) for informa-
tion on how to select the desired mode and Section 17.1.8
DC-coupled Input Signals and Section 17.1.7 AC-coupled In-
put Signals for applications information.
16.3.1.2 Input Full-Scale Range Adjust
The input full-scale range for the ADC10D1000/1500 may be
adjusted via Non-ECM or ECM. In Non-ECM, a control pin
selects a higher or lower value; see Section 16.2.1.9 Full-
Scale Input Range Pin (FSR). In ECM, the input full-scale
range may be adjusted with 15-bits of precision. See
VIN_FSR in Table 8 for electrical specification details. Note that
the higher and lower full-scale input range settings in Non-
ECM correspond to the mid and min full-scale input range
settings in ECM. It is necessary to execute an on-command
calibration following a change of the input full-scale range.
See Section 18.0 Register Definitions for information about
the registers.
16.3.1.3 Input Offset Adjust
The input offset adjust for the ADC10D1000/1500 may be
adjusted with 12-bits of precision plus sign via ECM. See
Section 18.0 Register Definitions for information about the
registers.
16.3.1.4 DES/Non-DES Mode
The ADC10D1000/1500 can operate in Dual-Edge Sampling
(DES) or Non-DES Mode. The DES Mode allows for one of
the ADC10D1000/1500's inputs to be sampled by both chan-
nels' ADCs. One ADC samples the input on the rising edge
of the sampling clock and the other ADC samples the same
input on the falling edge of the sampling clock. A single input
is thus sampled twice per clock cycle, resulting in an overall
sample rate of twice the sampling clock frequency, e.g.
2.0/3.0 GSPS with a 1.0/1.5 GHz sampling clock. See Sec-
tion 16.2.1.1 Dual Edge Sampling Pin (DES) for information
on how to select the desired mode. Since DES Mode uses
both I- and Q-channels to process the input signal, both chan-
nels must be powered up for the DES Mode to function
properly.
In Non-ECM, only the I-input may be used for the DES Mode
input. In ECM, either the I- or Q-input may be selected by first
using the DES bit (Addr: 0h, Bit 7) to select the DES Mode.
The DEQ Bit (Addr: 0h, Bit: 6) is used to select the Q-input,
but the I-input is used by default. Also, both I- and Q-inputs
may be driven externally, i.e. DESIQ Mode, by using the DIQ
bit (Addr: 0h, Bit 5). See Section 17.1 THE ANALOG IN-
PUTS for more information about how to drive the ADC in
DES Mode.
The DESIQ Mode results in the best bandwidth. In general,
the bandwidth decreases from Non-DES Mode to DES Mode
(specifically, DESI or DESQ) because both channels are
sampling off the same input signal and non-ideal effects in-
troduced by interleaving the two channels lower the band-
width. Driving both I- and Q-channels externally (DESIQ
Mode) results in better bandwidth for the DES Mode because
each channel is being driven, which reduces routing losses
(increases bandwidth).
In the DES Mode, the outputs must be carefully interleaved
in order to reconstruct the sampled signal. If the device is
programmed into the 1:4 Demux DES Mode, the data is ef-
fectively demultiplexed by 1:4. If the sampling clock is 1.0/1.5
GHz, the effective sampling rate is doubled to 2.0/3.0 GSPS
and each of the 4 output buses has an output rate of 500
MSPS. All data is available in parallel. To properly reconstruct
the sampled waveform, the four bytes of parallel data that are
output with each DCLK must be correctly interleaved. The
sampling order is as follows, from the earliest to the latest:
DQd, DId, DQ, DI. See Figure 7. If the device is programmed
into the Non-Demux DES Mode, two bytes of parallel data are
output with each edge of the DCLK in the following sampling
order, from the earliest to the latest: DQ, DI. See Figure 8.
The performance of the ADC10D1000/1500 in DES Mode
depends on how well the two channels are interleaved, i.e.
that the clock samples either channel with precisely a 50%
duty-cycle, each channel has the same offset (nominally code
511/512), and each channel has the same full-scale range.
The ADC10D1000/1500 includes an automatic clock phase
background adjustment in DES Mode to automatically and
continuously adjust the clock phase of the I- and Q-channels,
which also removes the need to adjust the clock phase setting
manually. A difference exists in the typical offset between the
I- and Q-channels, which can be removed via the offset adjust
feature in ECM, to optimize DES Mode performance. If pos-
sible, it is recommended to use the Q-input for better DES
Mode performance with no offset adjustment required. To ad-
just the I- or Q-channel offset, measure a histogram of the
digital data and adjust the offset via the Control Register until
the histogram is centered at code 511/512. Similarly, the full-
scale range of each channel may be adjusted for optimal
performance.
16.3.1.5 Sampling Clock Phase Adjust
The sampling clock (CLK) phase may be delayed internally to
the ADC up to 825 ps in ECM. This feature is intended to help
the system designer remove small imbalances in clock distri-
bution traces at the board level when multiple ADCs are used,
or to simplify complex system functions such as beam steer-
ing for phase array antennas.
Additional delay in the clock path also creates additional jitter,
so a clock jitter-cleaner is made available when using the
sampling clock phase adjust, see Section 16.3.1.6 LC Filter
on Sampling Clock. Nevertheless, because the sampling
clock phase adjust delays all clocks, including the DCLKs and
output data, the user is strongly advised to use the minimal
amount of adjustment and verify the net benefit of this feature
in his system before relying on it.
16.3.1.6 LC Filter on Sampling Clock
A LC bandpass filter is available on the ADC10D1000/1500
sampling clock to clean jitter on the incoming clock. This fea-
ture is only available when the CLK phase adjust feature is
also used. This feature was designed to minimize the dynamic
performance degradation resulting from additional clock jitter
as much as possible. It is available in ECM via the LCF (LC
Filter) bits in the Control Register (Addr: Dh, Bits 7:0).
If the clock phase adjust feature is enabled, the sampling
clock passes through additional gate delay, which adds jitter
to the clock signal. The LC filter helps to remove this additional
jitter, so it is only available when the clock phase adjust fea-
ture is also enabled. To enable both features, use SA (Addr:
Dh, Bit 8). The LCF bits are thermometer encoded and may
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ADC10D1000/1500