Datasheet

Data Sheet AD9742
Rev. C | Page 13 of 32
REFERENCE CONTROL AMPLIFIER
The AD9742 contains a control amplifier that is used to regulate
the full-scale output current, I
OUTFS
. The control amplifier is
configured as a V-I converter, as shown in Figure 24, so that its
current output, I
REF
, is determined by the ratio of the V
REFIO
and
an external resistor, R
SET
, as stated in Equation 4. I
REF
is copied
to the segmented current sources with the proper scale factor to
set I
OUTFS
, as stated in Equation 3.
The control amplifier allows a wide (10:1) adjustment span of
IOUTFS over a 2 mA to 20 mA range by setting IREF between
62.5 µA and 625 µA. The wide adjustment span of IOUTFS
provides several benefits. The first relates directly to the power
dissipation of the AD9742, which is proportional to IOUTFS (see
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 AD9742 provide complementary current
outputs, IOUTA and IOUTB. IOUTA provides a near full-scale
current output, I
OUTFS
, 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 I
OUTFS
and can be
expressed as:
( )
OUTFS
ICODEDACIOUTA ×= 4096/
(1)
( )
OUTFS
ICODEDACIOUTB ×−= /40964095
(2)
where DAC CODE = 0 to 4095 (i.e., decimal representation).
As mentioned previously, I
OUTFS
is a function of the reference
current I
REF
, which is nominally set by a reference voltage,
V
REFIO
, and external resistor, R
SET
. It can be expressed as:
REF
OUTFS
II ×= 32
(3)
where
SET
REFIO
REF
RVI /=
(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, R
LOAD
, that are tied to analog common, ACOM. Note that
R
LOAD
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
LOAD
OUTA
RIOUTAV ×=
(5)
LOAD
OUTB
RIOUTBV ×=
(6)
Note that the full-scale value of V
OUTA
and V
OUTB
should not exceed
the specified output compliance range to maintain specified
distortion and linearity performance.
( )
LOAD
DIFF
RIOUTBIOUTAV ×−=
(7)
Substituting the values of IOUTA, IOUTB, I
REF
, and V
DIFF
can be
expressed as:
( ){ }
( )
REFIO
SET
LOAD
DIFF
VRR
CODEDACV
××
−×=
/32
4096/40952
(8)
Equations 7 and 8 highlight some of the advantages of operating
the AD9742 differentially. First, the differential operation helps
cancel common-mode error sources associated with IOUTA
and IOUTB, such as noise, distortion, and dc offsets. Second,
the differential code-dependent current and subsequent voltage,
V
DIFF
, is twice the value of the single-ended voltage output (i.e.,
V
OUTA
or V
OUTB
), thus providing twice the signal power to the load.
Note that the gain drift temperature performance for a single-
ended (V
OU TA
and V
OUTB
) or differential output (V
DIFF
) of the
AD9742 can be enhanced by selecting temperature tracking
resistors for R
LOAD
and R
SET
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 comple-
mentary single-ended voltage outputs, V
OUTA
and V
OUTB
, via a
load resistor, R
LOAD
, as described in the DAC Transfer Function
section by Equations 5 through 8. The differential voltage, V
DIFF
,
existing between V
OUTA
and V
OUTB
, can also be converted to a
single-ended voltage via a transformer or differential amplifier
configuration. The ac performance of the AD9742 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.
The distortion and noise performance of the AD9742 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
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
waveform increases and/or its amplitude decreases. 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). Since the output currents of IOUTA and IOUTB
are complementary, they become additive when processed
differentially. A properly selected transformer will allow the
AD9742 to provide the required power and voltage levels to
different loads.