AD9238BSTRL-20 Analog Devices Inc, AD9238BSTRL-20 Datasheet - Page 19

• Manufacturer Part Number: AD9238BSTRL-20
• Description: IC ADC 12BIT DUAL 20MSPS 64-LQFP
• Manufacturer: Analog Devices Inc

Specifications of AD9238BSTRL-20

• Rohs Status: RoHS non-compliant
• Number Of Bits: 12
• Sampling Rate (per Second): 20M
• Data Interface: Parallel
• Number Of Converters: 2
• Power Dissipation (max): 180mW
• Voltage Supply Source: Single Supply
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 64-LQFP

DATA FORMAT
The AD9238 data output format can be configured for either
twos complement or offset binary. This is controlled by the data
format select pin (DFS). Connecting DFS to AGND produces
offset binary output data. Conversely, connecting DFS to AVDD
formats the output data as twos complement.
The output data from the dual ADCs can be multiplexed onto a
single 12-bit output bus. The multiplexing is accomplished by
toggling the MUX_SELECT bit, which directs channel data to
the same or opposite channel data port. When MUX_SELECT
is logic high, the Channel A data is directed to the Channel A
output bus, and the Channel B data is directed to the Channel B
output bus. When MUX_SELECT is logic low, the channel data
is reversed, that is the Channel A data is directed to the
Channel B output bus, and the Channel B data is directed to the
Channel A output bus. By toggling the MUX_SELECT bit,
multiplexed data is available on either of the output data ports.
If the ADCs run with synchronized timing, this same clock can
be applied to the MUX_SELECT pin. Any skew between CLK_A,
CLK_B, and MUX_SELECT can degrade ac performance. It is
recommended to keep the clock skew <100 pS. After the
MUX_SELECT rising edge, either data port has the data for its
respective channel; after the falling edge, the alternate channel’s
data is placed on the bus. Typically, the other unused bus would
be disabled by setting the appropriate OEB high to reduce
power consumption and noise. Figure 34 shows an example of
multiplex mode. When multiplexing data, the data rate is two
times the sample rate. Note that both channels must remain
active in this mode and that each channel’s power-down pin
must remain low.
VOLTAGE REFERENCE
A stable and accurate 0.5 V voltage reference is built into the
AD9238. The input range can be adjusted by varying the reference
voltage applied to the AD9238, using either the internal
reference with different external resistor configurations or an
externally applied reference voltage. The input span of the ADC
tracks reference voltage changes linearly. If the ADC is being
driven differentially through a transformer, the reference voltage
can be used to bias the center tap (common-mode voltage).
The shared reference mode allows the user to connect the references
from the dual ADCs together externally for superior gain and
offset matching performance. If the ADCs are to function
independently, the reference decoupling can be treated
Table 7. Reference Configuration Summary
Selected Mode
External Reference
Internal Fixed Reference
Programmable Reference
Internal Fixed Reference
SENSE Voltage
AVDD
VREF
0.2 V to VREF
AGND to 0.2 V
Rev. C | Page 19 of 48
Resulting VREF (V)
N/A
0.5
0.5 × (1 + R2/R1)
1.0
independently and can provide superior isolation between the dual
channels. To enable shared reference mode, the SHARED_REF
pin must be tied high and the external differential references
must be externally shorted. (REFT_A must be externally
shorted to REFT_B, and REFB_A must be shorted to REFB_B.)
Internal Reference Connection
A comparator within the AD9238 detects the potential at the
SENSE pin and configures the reference into four possible states,
which are summarized in Table 7. If SENSE is grounded, the
reference amplifier switch is connected to the internal resistor
divider (see Figure 35), setting VREF to 1 V. Connecting the
SENSE pin to VREF switches the reference amplifier output to
the SENSE pin, completing the loop and providing a 0.5 V
reference output. If a resistor divider is connected, as shown in
Figure 36, the switch is again set to the SENSE pin. This puts
the reference amplifier in a noninverting mode with the VREF
output defined as
In all reference configurations, REFT and REFB drive the ADC
core and establish its input span. The input range of the ADC
always equals twice the voltage at the reference pin for either an
internal or an external reference.
VREF = 0.5 × (1 + R2/R1)
10μF
SENSE
VIN+
VIN–
0.1μF
VREF
Figure 35. Internal Reference Configuration
Resulting Differential Span (V p-p)
2 × External Reference
1.0
2 × VREF (See Figure 36)
2.0
SELECT
LOGIC
AD9238
0.5V
CORE
ADC
REFB
REFT
0.1μF
0.1μF
0.1μF
AD9238
10μF