AD9765(RevB) Ver la hoja de datos (PDF) - Analog Devices
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AD9765 Datasheet PDF : 28 Pages
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AD9765
MASTER/SLAVE RESISTOR MODE, GAINCTRL
The AD9765 allows the gain of each channel to be indepen-
dently set by connecting one RSET resistor to FSADJ1 and
another RSET resistor to FSADJ2. To add flexibility and reduce
system cost, a single RSET resistor can be used to set the gain of
both channels simultaneously.
When GAINCTRL is low (i.e., connected to AGND), the inde-
pendent channel gain control mode using two resistors is enabled.
In this mode, individual RSET resistors should be connected to
FSADJ1 and FSADJ2. When GAINCTRL is high (i.e., con-
nected to AVDD), the master/slave channel gain control mode
using one resistor is enabled. In this mode, a single RSET resistor
is connected to FSADJ1 and the resistor on FSADJ2 must be
removed.
NOTE: Only parts with date code of 9930 or later have the
Master/Slave GAINCTRL function. For parts with a date code
before 9930, Pin 42 must be connected to AGND, and the part
will operate in the two resistor, independent gain control mode.
REFERENCE CONTROL AMPLIFIER
Both of the DACs in the AD9765 contain a control amplifier
that is used to regulate the full-scale output current, IOUTFS.
The control amplifier is configured as a V-I converter as shown
in Figure 21, so that its current output, IREF, is determined
by the ratio of the VREFIO and an external resistor, RSET, as
stated in Equation 4. IREF is copied to the segmented current
sources with the proper scale factor to set IOUTFS as stated in
Equation 3.
The control amplifier allows a wide (10:1) adjustment span of
IOUTFS from 2 mA to 20 mA by setting IREF between 62.5 µA
and 625 µA. The wide adjustment range of IOUTFS provides
several benefits. The first relates directly to the power dissipa-
tion of the AD9765, which is proportional to IOUTFS (refer to
the Power Dissipation section). The second relates to the 20 dB
adjustment, which is useful for system gain control purposes.
The small signal bandwidth of the reference control amplifier is
approximately 500 kHz and can be used for low frequency,
small signal multiplying applications.
DAC TRANSFER FUNCTION
Both DACs in the AD9765 provide complementary current
outputs, IOUTA and IOUTB. IOUTA will provide a near full-scale
current output, IOUTFS, when all bits are high (i.e., DAC CODE
= 4095) while IOUTB, the complementary output, provides no
current. The current output appearing at IOUTA and IOUTB
is a function of both the input code and IOUTFS and can be
expressed as:
IOUTA = (DAC CODE/4096) × IOUTFS
(1)
IOUTB = (4095 – DAC CODE/4096) × IOUTFS
(2)
where DAC CODE = 0 to 4095 (i.e., Decimal Representation).
As previously mentioned, IOUTFS is a function of the reference
current IREF, which is nominally set by a reference voltage,
VREFIO and external resistor RSET. It can be expressed as:
IOUTFS = 32 × IREF
(3)
where
IREF = VREFIO /RSET
(4)
The two current outputs will typically drive a resistive load
directly or via a transformer. If dc coupling is required, IOUTA
and IOUTB should be directly connected to matching resistive
loads, RLOAD, that are tied to analog common, ACOM. Note,
RLOAD may represent the equivalent load resistance seen by
IOUTA or IOUTB as would be the case in a doubly terminated 50
Ω or 75 Ω cable. The single-ended voltage output appearing at
the IOUTA and IOUTB nodes is simply:
VOUTA = IOUTA × RLOAD
(5)
VOUTB = IOUTB × RLOAD
(6)
Note the full-scale value of VOUTA and VOUTB should not exceed
the specified output compliance range to maintain specified
distortion and linearity performance.
VDIFF = (IOUTA – IOUTB) × RLOAD
(7)
Substituting the values of IOUTA, IOUTB and IREF; VDIFF can be
expressed as:
VDIFF = {(2 × DAC CODE – 4095)/4096} ×
(32 × RLOAD/RSET) × VREFIO
(8)
These last two equations highlight some of the advantages of
operating the AD9765 differentially. First, the differential
operation will help cancel common-mode error sources asso-
ciated with IOUTA and IOUTB such as noise, distortion and dc
offsets. Second, the differential code-dependent current and
subsequent voltage, VDIFF, is twice the value of the single-ended
voltage output (i.e., VOUTA or VOUTB), thus providing twice the
signal power to the load.
Note, the gain drift temperature performance for a single-ended
(VOUTA and VOUTB) or differential output (VDIFF) of the AD9765
can be enhanced by selecting temperature tracking resistors for
RLOAD and RSET due to their ratiometric relationship as shown in
Equation 8.
ANALOG OUTPUTS
The complementary current outputs in each DAC, IOUTA and
IOUTB, may be configured for single-ended or differential operation.
IOUTA and IOUTB can be converted into complementary single-
ended voltage outputs, VOUTA and VOUTB, via a load resistor,
RLOAD, as described in the DAC Transfer Function section by
Equations 5 through 8. The differential voltage, VDIFF, existing
between VOUTA and VOUTB can also be converted to a single-ended
voltage via a transformer or differential amplifier configuration.
The ac performance of the AD9765 is optimum and specified
using a differential transformer coupled output in which the
voltage swing at IOUTA and IOUTB is limited to ± 0.5 V. If a single-
ended unipolar output is desirable, IOUTA should be selected.
The distortion and noise performance of the AD9765 can be
enhanced when it is configured for differential operation. The
common-mode error sources of both IOUTA and IOUTB can be
significantly reduced by the common-mode rejection of a trans-
former or differential amplifier. These common-mode error
sources include even-order distortion products and noise. The
enhancement in distortion performance becomes more signifi-
cant as the frequency content of the reconstructed waveform
increases. This is due to the first order cancellation of various
dynamic common-mode distortion mechanisms, digital feed-
through and noise.
–10–
REV. B
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