AD668SQ883B Analog Devices, AD668SQ883B Datasheet - Page 5

• Manufacturer Part Number: AD668SQ883B
• Description: The AD668 is an ultrahigh speed, 12-bit, multiplying digital-to-analog converter, providing outstanding accuracy and ...
• Manufacturer: Analog Devices

REV. A
ANALOG OFFSET ERROR: The analog offset is defined as
the offset of the analog amplifier channel, referred to the analog
input. Ideally, this would be measured with the analog input at
0 V and the digital input at full scale. Since a 0 V analog input
voltage constitutes an undervoltage condition, this specification
is determined through linear extrapolation, as indicated in
Figure 1.
GLITCH IMPULSE: Asymmetrical switching times in a DAC
may give rise to undesired output transients which are quanti-
fied by their glitch impulse. It is specified as the net area of the
glitch in pV-sec.
FUNCTIONAL DESCRIPTION
The AD668 is designed to combine excellent performance with
maximum flexibility. The functional block diagram and the
simple transfer functions provided below will provide the user
with a basic grasp of the AD668’s operation. Examples of typi-
cal circuit configurations are provided in the section APPLY-
ING THE AD668. Subsequent sections contain more detailed
information useful in optimizing DAC performance in high
speed, high resolution applications.
DAC Transfer Function
The AD668 may be used either in a current output mode (DAC
output connected to a virtual ground) or a voltage output mode
(DAC output connected to a resistive load).
Figure 1. Derivation of Analog Offset Voltage
Figure 2. AD668 Major Carry Glitch
–5–
In current output mode:
Unipolar Mode
Bipolar Mode
In voltage output mode:
(for both unipolar and bipolar modes)
where:
V
V
1.25 V, or 5 V, determined by the wiring configuration of Pins
21 and 22. (See APPLYING THE AD668.)
DAC code – the numerical representation of the DAC’s digital
inputs; a number between 0 and 4095.
R
this can be is 200 Ω (the internal DAC ladder resistance). The
on-board load resistor (Pin 19) has been trimmed so that its
parallel combination with the DAC ladder resistance is 100 Ω
(± 1%)
Bipolar mode – produces a bipolar analog output from the digital
input by offsetting the normal output current with a precision
current source. This offset is achieved by connecting Pin 16 to
the DAC output. In the unipolar mode, Pin 16 should be
grounded.
If the dc errors are included, the transfer function becomes
somewhat more complex:
(Last term is for use in bipolar mode; V
R
where:
OFFSET
OFFSET
OFFSET
E = the gain error, expressed fractionally.
Operating Limits:
I
I
–
OUT
OUT



IN
NOM
LOAD
LOAD
V
V
– the analog input voltage.
NOM
=
IN
– the nominal full scale of the reference voltage: 1 V,
=



)
– the resistance of the DAC output node; the maximum
V
V
V
+ OFFSET
+ OFFSET
NOM
ANALOG
DIGITAL
BIPOLAR
V
I
IN
NOM
OUT
IN
+ OFFSET
=
×
= is the unipolar digital offset error.
= the analog offset error.
= is the bipolar offset error.
DIGITAL
V
ANALOG
DAC code
V
V
NOM
OUT
IN
4096
ANALOG



×
×
= I
V
× (5.12 mA + [OFFSET
V
NOM
DAC code
IN
OUT



× 10.24 mA –
4096
×
× 10.24 mA
DAC code
× R
4096
LOAD
× 10.24 mA
OUT
× (1 + E ) × 10.24 mA
BIPOLAR
is still just I
V
V
NOM
IN
AD668
× 10.24 mA])
× 5.12 mA
OUT
×