ANALOGDEVICES fAX-ON-DEMAND HOTLINE -Page 21
PM-7543
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FIGURE 3: PM-7543 Equivalent Circuit (All Inputs LOW)
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FIGURE4: PM- 7543 Equivalent Circuit (AllDigital Inputs HIGH)
EQUIVALENTCIRCUITANALVSIS
Figures 3 and 4 show equivalent circuits for the PM-7543's inter-
nal DAC with all bits LOW and HIGH, respectively. The refer-
ence current is switched to IOUT2when all data bits are LOW,
and to lOUT' when aU bits are HIGH. The ILEAKAGEcurrent
source is the combination of surface and junction leakages to
the substrate. The 1/4096 current source represents the con-
stant 1-bit current drain through the ladder's terminating resis-
tor.
Output capacitance is dependent upon thedigital input code.
This is because the gate capacitance of MOS transistors in-
creases with applied gate voltage. This output capacitance var-
ies between the low and high values.
DYNAMIC PERFORMANCE
OUTPUT IMPEDANCE
The output resistance, as in the case ofthe output capacitance,
varies with the digital input code. This resistance, looking back
into the 'OUT1terminal, may be between 11kn (the feedback
resistor alone when all digital inputs are LOW) and 7.5kQ (the
feedback resistor in parallel with approximately 30kQ of the R-
2R ladder network resistance when any single bit logic is HIGH).
Static accuracy and dynamic performance will be affected by
these variations.
The gain and phase stability of the output amplifier, board lay-
out, and power supply decoupling will all affect the dynamic
performance of the PM-7543. The use of a small compensation
capacitor may be required when high-speed operational ampli-
fiers are used. It may be connected across the amplifiers feed-
back resistor to provide the necessary phase compensation to
critically damp the output.
The considerations when using high-speed amplifiers are:
1. Phase compensation (see Figures 7 and 8).
2. Power supply decoupling at the device socket and use of
proper grounding techniques.
APPLICATIONS INFORMATION
APPLICATIONTIPS
In most applications, linearity depends upon the potential of
IOUT1''OUT2'and AGND (pins 1,2, and 3) being exactly equal to
each other.Inmostapplications,the DACis connected to an
external op amp with its noninverting input tied to ground (see
Figures 7 and 8). The amplifier selected should have a low input
bias current and low drift over temperature. The amplifier's input
offset voltage should be nulled to less than :t200~V (less than
10% of 1 LSB).
The operational amplifier's non inverting input should have a
minimum resistance connection to ground; the usual bias cur-
rent compensation resistor should not be used. This resistor can
cause a variable offset voltage appearing as a varying output
error. Allgrounded pins should tie to a single common ground
point, avoiding ground loops. The VOD power supply should
have a low noise level with no transients greater than +17V.
It is recommended that the digital inputs be taken to ground or
VDOvia a high value (1Mn) resistor; this will prevent the accu-
mulation of static charge if the PC card is disconnected from the
system.
Peak supply current flows as the digital inputs pass through the
transition region (see the Supply Current vs logic Input Voltage
graph under the Typical Performance Characteristics). The
supply current decreases as the input voltage approaches the
supply rails (Voo or DGND), i.e. rapidly slewing logic signals that
settle very near the supply rails will minimize supply current.
OUTPUT AMPLIFIER CONSIDERATIONS
When using high speed op amps, a small feedback capacitor
(typically 5-30pF) should be used across the amplifier to mini-
mize overshoot and ringing, Forlow speed or static applications,
AC specifications of the amp1ifier are not very critical. In high-
speed applications, slew rate, settling time, open-loop gain, and
gain/phase margin specifications of the amplifier should be se-
lected forthe desired performance. Ithas already been noted
that an offset can be caused by including the usual bias current
compensation resistor in the amplifier's non inverting input ter-
minal. This resistor should not be used. Instead, the amplifier
should have a bias current which is low over the temperature
range of interest.
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