AD9238BCP40EB Analog Devices Inc, AD9238BCP40EB Datasheet - Page 19

• Manufacturer Part Number: AD9238BCP40EB
• Manufacturer: Analog Devices Inc

Specifications of AD9238BCP40EB

• Lead Free Status / Rohs Status: Not Compliant

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 33 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
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. B | Page 19 of 48
Resulting VREF (V)
N/A
0.5
0.5 × (1 + R2/R1)
1.0
gain and offset matching performance. If the ADCs are to
function independently, the reference decoupling can be
treated 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 34), 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 35, 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 34. Internal Reference Configuration
Resulting Differential Span (V p-p)
2 × External Reference
1.0
2 × VREF (See Figure 35)
2.0
SELECT
LOGIC
AD9238
0.5V
CORE
ADC
REFB
REFT
0.1µF
0.1µF
0.1µF
AD9238
10µF