AD668 데이터 시트보기 (PDF) - Analog Devices
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AD668 Datasheet PDF : 16 Pages
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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.
Figure 1. Derivation of Analog Offset Voltage
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.
AD668
In current output mode:
Unipolar Mode
IOUT
= V IN
V NOM
×
DAC code
4096
× 10.24 mA
Bipolar Mode
IOUT
= V IN
V NOM
×
DAC code
4096
× 10.24 mA – V IN
V NOM
× 5.12 mA
In voltage output mode:
VOUT = IOUT × RLOAD
(for both unipolar and bipolar modes)
where:
VIN – the analog input voltage.
VNOM – the nominal full scale of the reference voltage: 1 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.
RLOAD – the resistance of the DAC output node; the maximum
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:
IOUT
=
VIN
VNOM
DAC code
+ OFFSET ANALOG × 4096 × (1 + E ) × 10.24 mA
Figure 2. AD668 Major Carry Glitch
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).
+ OFFSET DIGITAL × VIN × 10.24 mA
VNOM
–
V IN
V NOM
+ OFFSET ANALOG
× (5.12 mA + [OFFSET BIPOLAR
× 10.24 mA])
(Last term is for use in bipolar mode; VOUT is still just IOUT ×
RLOAD)
where:
OFFSETANALOG = the analog offset error.
OFFSETDIGITAL = is the unipolar digital offset error.
OFFSETBIPOLAR = is the bipolar offset error.
E = the gain error, expressed fractionally.
Operating Limits:
REV. A
–5–
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