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AD7823 数据表(PDF) 7 Page - Analog Devices

部件名 AD7823
功能描述  2.7 V to 5.5 V, 5s, 8-Bit ADC in 8-Lead microSOIC/DIP
Download  11 Pages
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制造商  AD [Analog Devices]
网页  http://www.analog.com
标志 AD - Analog Devices

AD7823 数据表(HTML) 7 Page - Analog Devices

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AD7823
–7–
REV. C
When using the pseudo differential input scheme the signal on
VIN– must not vary by more than a 1/2 LSB during the conver-
sion process. If the signal on VIN– varies during conversion, the
conversion result will be incorrect. For single ended operation, VIN–
is always connected to AGND. Figure 9 shows the AD7823 pseudo
differential input being used to make a unipolar dc current mea-
surement. A sense resistor is used to convert the current to a
voltage, and the voltage is applied to the differential input as
shown.
VDD
RSENSE
RL
VIN+
VIN–
AD7823
Figure 9. DC Current Measurement Scheme
DC Acquisition Time
The ADC starts a new acquisition phase at the end of a conver-
sion and ends on the falling edge of the
CONVST signal. At the
end of a conversion there is a settling time associated with the
sampling circuit. This settling time lasts approximately 100 ns.
The analog signal on VIN+ is also being acquired during this
settling time; therefore, the minimum acquisition time needed is
approximately 100 ns.
Figure 10 shows the equivalent charging circuit for the sampling
capacitor when the ADC is in its acquisition phase. R2 represents
the source impedance of a buffer amplifier or resistive network;
R1 is an internal multiplexer resistance and C1 is the sampling
capacitor.
C1
3.5 F
R1
125
VIN+
R2
Figure 10. Equivalent Sampling Circuit
During the acquisition phase, the sampling capacitor must be
charged to within a 1/2 LSB of its final value. The time it takes
to charge the sampling capacitor (tCHARGE) is given by the follow-
ing formula:
tCHARGE = 6.2
× (R2 + 125 Ω) × 3.5 pF
For small values of source impedance, the settling time associated
with the sampling circuit (100 ns) is, in effect, the acquisition
time of the ADC. For example, with a source impedance (R2)
of 10
Ω, the charge time for the sampling capacitor is approxi-
mately 2 ns. The charge time becomes significant for source
impedances of 4.6 k
Ω and greater.
AC Acquisition Time
In ac applications it is recommended to always buffer analog
input signals. The source impedance of the drive circuitry must
be kept as low as possible to minimize the acquisition time of
the ADC. Large values of source impedance will cause the THD
to degrade at high throughput rates. In addition, better perfor-
mance can generally be achieved by using an external 1 nF
capacitor on VIN+.
ADC TRANSFER FUNCTION
The output coding of the AD7823 is straight binary. The designed
code transitions occur at successive integer LSB values (i.e.,
1 LSB, 2 LSBs, etc.). The LSB size is = VREF/256. The ideal trans-
fer characteristic for the AD7823 is shown in Figure 11 below.
000...010
000...001
000...000
0V
+VREF –1LSB
ANALOG INPUT
111...111
111...110
111...000
011...111
1LSB
1LSB = VREF/256
Figure 11. Transfer Characteristic


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