Datasheet

AD9714/AD9715/AD9716/AD9717
Rev. A | Page 44 of 80
DAC TRANSFER FUNCTION
The AD9714/AD9715/AD9716/AD9717 provide two differen-
tial current outputs, IOUTP/IOUTN and QOUTP/QOUTN.
IOUTP and QOUTP provide a near full-scale current output,
I
xOUTFS
, when all bits are high (that is, DAC CODE = 2
N
− 1,
where N = 8, 10, 12, or 14 for the AD9714, AD9715, AD9716,
and AD9717, respectively), while IOUTN and QOUTN, the
complementary outputs, provide no current. The current
outputs appearing at the positive DAC outputs, IOUTP and
QOUTP, and at the negative DAC outputs, IOUTN and QOUTN,
are a function of both the input code and I
xOUTFS
and can be
expressed as follows:
IOUTP = (IDAC CODE/2
N
) × I
IOUTFS
(1)
QOUTP = (QDAC CODE/2
N
) × I
QOUTFS
IOUTN = ((2
N
− 1) − IDAC CODE)/2
N
× I
IOUTFS
(2)
QOUTN = ((2
N
− 1) − QDAC CODE)/2
N
× I
QOUTFS
where:
IDAC CODE and QDAC CODE = 0 to 2
N
− 1 (that is, decimal
representation).
I
IOUTFS
and I
QOUTFS
are functions of the reference currents, I
IREF
and I
QREF
, respectively, which are nominally set by a reference
voltage, V
REFIO
, and external resistors, IR
SET
and QR
SET
, respec-
tively. I
IOUTFS
and I
QOUTFS
can be expressed as follows:
I
IOUTFS
= 32 × I
IREF
(3)
I
QOUTFS
= 32 × I
QREF
where:
I
IREF
= V
REFIO
/IR
SET
(4)
I
QREF
= V
REFIO
/QR
SET
or
I
IOUTFS
= 32 × V
REFIO
/IR
SET
(5)
I
QOUTFS
= 32 × V
REFIO
/QR
SET
A differential pair (IOUTP/IOUTN or QOUTP/QOUTN)
typically drives a resistive load directly or via a transformer. If
dc coupling is required, the differential pair (IOUTP/IOUTN or
QOUTP/QOUTN) should be connected to matching resistive
loads, xR
LOAD
, that are tied to analog common, AVSS. The
single-ended voltage output appearing at the positive and
negative nodes is
V
IOUTP
= IOUTP × IR
LOAD
(6)
V
QOUTP
= QOUTP × QR
LOAD
V
IOUTN
= IOUTN × IR
LOAD
(7)
V
QOUTN
= QOUTN × QR
LOAD
To achieve the maximum output compliance of 1 V at the
nominal 4 mA output current, IR
LOAD
= QR
LOAD
must be set
to 250 .
Substituting the values of IOUTP, IOUTN, and I
xREF
, V
IDIFF
can
be expressed as
V
IDIFF
= {(2 × IDAC CODE – (2
N
− 1))/2
N
} × (8)
(32 × V
REFIO
/IR
SET
) × IR
LOAD
Equation 8 highlights some of the advantages of operating the
AD9714/AD9715/AD9716/AD9717 differentially. First, the
differential operation helps cancel common-mode error sources
associated with IOUTP and IOUTN, such as noise, distortion,
and dc offsets. Second, the differential code-dependent current and
subsequent voltage, V
IDIFF
, is twice the value of the single-ended
voltage output (that is, V
IOUTP
or V
IOUTN
), thus providing twice
the signal power to the load. Note that the gain drift temperature
performance for a single-ended output (V
IOUTP
and V
IOUTN
) or
differential output (V
IDIFF
) of the AD9714/AD9715/AD9716/
AD9717 can be enhanced by selecting temperature-tracking
resistors for xR
LOAD
and xR
SET
because of their ratiometric
relationship, as shown in Equation 8.
ANALOG OUTPUT
The complementary current outputs in each DAC, IOUTP/
IOUTN and QOUTP/QOUTN, can be configured for single-
ended or differential operation. IOUTP/IOUTN and QOUTP/
QOUTN can be converted into complementary single-ended
voltage outputs, V
IOUTP
and V
IOUTN
, as well as V
QOUTP
and V
QOUTN
via a load resistor, xR
LOAD
, as described in the DAC Transfer
Function section by Equation 6 through Equation 8. The differen-
tial voltages, V
IDIFF
and V
QDIFF
, existing between V
IOUTP
and V
IOUTN
,
and V
QOUTP
and V
QOUTN
, can also be converted to a single-ended
voltage via a transformer or a differential amplifier configuration.
The ac performance of the AD9714/AD9715/AD9716/AD9717
is optimum and is specified using a differential transformer-
coupled output in which the voltage swing at IOUTP and IOUTN
is limited to ±0.5 V. The distortion and noise performance of
the AD9714/AD9715/AD9716/AD9717 can be enhanced when
it is configured for differential operation. The common-mode
error sources of both IOUTP/IOUTN and QOUTP/QOUTN
can be significantly reduced by the common-mode rejection
of a transformer or differential amplifier. These common-mode
error sources include even-order distortion products and noise.
The enhancement in distortion performance becomes more
significant as the frequency content of the reconstructed wave-
form increases and/or its amplitude increases. This is due to
the first-order cancellation of various dynamic common-mode
distortion mechanisms, digital feedthrough, and noise. Performing
a differential-to-single-ended conversion via a transformer also
provides the ability to deliver twice the reconstructed signal
power to the load (assuming no source termination). Because
the output currents of IOUTP/IOUTN and QOUTP/QOUTN
are complementary, they become additive when processed
differentially.