AD9705 Analog Devices, AD9705 Datasheet - Page 35

• Manufacturer Part Number: AD9705
• Manufacturer: Analog Devices

Specifications of AD9705

• Resolution (bits): 10bit
• Dac Update Rate: 175MSPS
• Dac Settling Time: 11ns
• Max Pos Supply (v): +3.6V
• Single-supply: Yes
• Dac Type: Current Out
• Dac Input Format: Par

Data Sheet
DAC TRANSFER FUNCTION
The
current outputs, IOUTA and IOUTB. IOUTA provides a near
full-scale current output, I
DAC CODE = 2
AD9705, AD9706, and AD9707, respectively), 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
where DAC CODE = 0 to 2
I
nominally set by a reference voltage, V
resistor, R
where
The two current outputs typically drive a resistive load directly
or via a transformer. If dc coupling is required, IOUTA and
IOUTB should be connected to matching resistive loads (R
that are tied to analog common (ACOM). The single-ended
voltage output appearing at the IOUTA and IOUTB nodes is
To achieve the maximum output compliance of 1 V at the
nominal 2 mA output current, R
Also, the full-scale value of V
the specified output compliance range to maintain specified
distortion and linearity performance.
Substituting the values of IOUTA, IOUTB, I
expressed as
Equation 7 and Equation 8 highlight some of the advantages of
operating the
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
single-ended voltage output (that is, V
providing twice the signal power to the load.
The gain drift temperature performance for a single-ended
output (V
the
selecting temperature tracking resistors for R
because of their ratiometric relationship, as shown in Equation 8.
OUTFS
AD9704/AD9705/AD9706/AD9707
AD9704/AD9705/AD9706/AD9707
IOUTA = (DAC CODE/2
IOUTB = ((2
I
I
V
V
V
V
(32 × V
OUTFS
REF
is a function of the reference current, I
IOUTA
IOUTB
DIFF
DIFF
= V
IOUTA
OUTFS
SET
= (IOUTA – IOUTB) × R
= {(2 × DAC CODE – (2
= 32 × I
= IOUTB × R
= IOUTA × R
. It can be expressed as
REFIO
REFIO
AD9704/AD9705/AD9706/AD9707
and can be expressed as
and V
N
/R
/R
− 1, where N = 8, 10, 12, or 14 for the AD9704,
N
REF
SET
SET
− 1) − DAC CODE)/2
IOUTB
) × R
LOAD
LOAD
) or the differential output (V
LOAD
N
OUTFS
− 1 (that is, decimal representation).
IOUTA
N
, when all bits are high (that is,
) × I
LOAD
and V
DIFF
N
OUTFS
LOAD
− 1))/2
must be set to 500 Ω.
, is twice the value of the
IOUTA
REFIO
can be enhanced by
provide complementary
IOUTB
N
× I
, and an external
N
or V
} ×
REF
REF
LOAD
must not exceed
OUTFS
, and V
, which is
IOUTB
and R
differentially.
), thus
DIFF
DIFF
SET
LOAD
can be
,
) of
)
Rev. B | Page 35 of 44
(1)
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(8)
ANALOG OUTPUTS
The complementary current outputs in each DAC, IOUTA, and
IOUTB can be configured for single-ended or differential oper-
ation. IOUTA and IOUTB can be converted into complementary
single-ended voltage outputs, V
R
Equation 5 through Equation 8. The differential voltage, V
existing between V
single-ended voltage via a transformer or a differential amplifier
configuration. The ac performance of the AD9704/AD9705/
AD9706/AD9707
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 AD9704/AD9705/
AD9706/AD9707
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 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 IOUTA and
IOUTB are complementary, they become additive when
processed differentially.
When the
nominal operating point of 2 mA output current and 0.5 V output
swing is desired, R
transformer allows the AD9704/AD9705/AD9706/
provide the required power and voltage levels to different loads.
The output impedance of IOUTA and IOUTB is determined by
the equivalent parallel combination of the PMOS switches
associated with the current sources and is typically 200 MΩ in
parallel with 5 pF. It is also slightly dependent on the output
voltage (that is, V
device. As a result, maintaining IOUTA and/or IOUTB at a
virtual ground via an I-V op amp configuration results in the
optimum dc linearity. Note that the INL/DNL specifications for
the
maintained at a virtual ground via an op amp.
IOUTA and IOUTB also have a negative and positive voltage
compliance range that must be adhered to in order to achieve
optimum performance. The absolute maximum negative output
compliance range of −1 V is set by the breakdown limits of the
CMOS process. Operation beyond this maximum limit can result
in a breakdown of the output stage and affect the reliability of
the AD9704/AD9705/AD9706/AD9707.
LOAD
AD9704/AD9705/AD9706/AD9707
, as described in the DAC Transfer Function section by
AD9704/AD9705/AD9706/AD9707
AD9704/AD9705/AD9706/AD9707
is optimum and is specified using a differential
IOUTA
can be enhanced when it is configured for
LOAD
IOUTA
and V
must be set to 250 Ω. A properly selected
and V
IOUTB
IOUTA
IOUTB
) due to the nature of a PMOS
, can also be converted to a
and V
are measured with IOUTA
IOUTB
, via a load resistor,
is being used at its
AD9707
to
DIFF
,