Analog Devices AD9740 Evaluation Board AD9740ACP-PCBZ AD9740ACP-PCBZ Fiche De Données
Codes de produits
AD9740ACP-PCBZ
AD9740
Rev. B | Page 14 of 32
150pF
1.2V REF
AVDD
REFLO
CURRENT
SOURCE
ARRAY
REFIO
FS ADJ
AD9740
REFERENCE
CONTROL
AMPLIFIER
3.3V
02
9
1
1-
0
23
Figure 25. External Reference Configuration
REFERENCE CONTROL AMPLIFIER
The AD9740 contains a control amplifier that is used to regulate
the full-scale output current, I
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
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
set I
OUTFS
, as stated in Equation 3.
The control amplifier allows a wide (10:1) adjustment span of
I
I
OUTFS
over a 2 mA to 20 mA range by setting I
REF
between
62.5 μA and 625 μA. The wide adjustment span of I
OUTFS
provides several benefits. The first relates directly to the power
dissipation of the AD9740, which is proportional to I
dissipation of the AD9740, which is proportional to I
OUTFS
(see
the Power Dissipation section). The second relates to a 20 dB
adjustment, which is useful for system gain control purposes.
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.
approximately 500 kHz and can be used for low frequency small
signal multiplying applications.
DAC TRANSFER FUNCTION
The AD9740 provides complementary current outputs, IOUTA
and IOUTB. IOUTA provides a near full-scale current output,
I
and IOUTB. IOUTA provides a near full-scale current output,
I
OUTFS
, when all bits are high (that is, DAC CODE = 1023), 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
current output appearing at IOUTA and IOUTB is a function of
both the input code and I
OUTFS
and can be expressed as:
IOUTA = (DAC CODE/1023) × I
OUTFS
(1)
IOUTB = (1023 − DAC CODE)/1024 × I
OUTFS
where DAC CODE = 0 to 1023 (that is, 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:
I
OUTFS
= 32 × I
REF
(3)
where
I
REF
= V
REFIO
/R
SET
(4)
The two current outputs typically drive a resistive load directly
or via a transformer. If dc coupling is required, then IOUTA
and IOUTB should be directly connected to matching resistive
loads, R
or via a transformer. If dc coupling is required, then IOUTA
and IOUTB should be directly connected to matching resistive
loads, R
LOAD
, that are tied to analog common, ACOM. Note that
R
LOAD
can 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
50 Ω or 75 Ω cable. The single-ended voltage output appearing
at the IOUTA and IOUTB nodes is simply
V
OUTA
= IOUTA × R
LOAD
(5)
V
OUTB
= IOUTB × R
LOAD
(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.
specified distortion and linearity performance.
V
DIFF
= (IOUTA − IOUTB) × R
LOAD
(7)
Substituting the values of IOUTA, IOUTB, I
REF
, and V
DIFF
can be
expressed as:
V
DIFF
= {(2 × DAC CODE − 1023)/1024}
(32 × R
LOAD
/R
SET
) × V
REFIO
(8)
Equation 7 and Equation 8 highlight some of the advantages of
operating the AD9740 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
operating the AD9740 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 (that is, 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
ended (V
OUTA
and V
OUTB
) or differential output (V
B
DIFF
) of the
AD9740 can be enhanced by selecting temperature tracking
resistors for R
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 can be configured for single-ended or differential
operation. IOUTA and IOUTB can be converted into
complementary single-ended voltage outputs, V
and IOUTB can be configured for single-ended or differential
operation. IOUTA and IOUTB can be converted into
complementary single-ended voltage outputs, V
OUTA
and V
OUTB
,
via a load resistor, R
LOAD
, as described in the DAC Transfer
Function section by Equation 5 through Equation 8. The
differential voltage, V
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
AD9740 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.
differential amplifier configuration. The ac performance of the
AD9740 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 AD9740 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
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