AD7816-7818 Analog Devices, AD7816-7818 Datasheet - Page 11

• Manufacturer Part Number: AD7816-7818
• Description: Single- and 4-Channel/ 9 us/ 10-Bit ADCs with On-Chip Temperature Sensor
• Manufacturer: Analog Devices

REV. A
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 a settling time is associated with the sam-
pling circuit. This settling time lasts approximately 100 ns. The
analog signal on V
time. Therefore, the minimum acquisition time needed is ap-
proximately 100 ns.
Figure 8 shows the equivalent charging circuit for the sampling
capacitor when the ADC is in its acquisition phase. R2 repre-
sents the source impedance of a buffer amplifier or resistive
network, R1 is an internal multiplexer resistance and C1 is the
sampling capacitor.
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 (T
following formula:
For small values of source impedance, the settling time associ-
ated with the sampling circuit (100 ns) is, in effect, the acquisi-
tion time of the ADC. For example with a source impedance
(R2) of 10
proximately 23 ns. The charge time becomes significant for
source impedances of 1 k 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.
ON-CHIP REFERENCE
The AD7816, AD7817 and AD7818 have an on-chip +1.2 V
bandgap reference that is gained up to give an output of +2.5 V.
The on-chip reference is selected by connecting the REF
to analog ground. This causes SW1 (see Figure 9) to open and
the reference amplifier to power up during a conversion. There-
fore the on-chip reference is not available externally. An ex-
ternal +2.5 V reference can be connected to the REF
This has the effect of shutting down the on-chip reference cir-
cuitry and reducing I
A
IN
Figure 7. Equivalent Analog Input Circuit
4pF
C2
Figure 8. Equivalent Sampling Circuit
R2
the charge time for the sampling capacitor is ap-
T
CHARGE
IN +
V
DD
DD
is also being acquired during this settling
= 7.6
D1
D2
by about 0.25 mA.
V
IN
CONVERT PHASE - SWITCH OPEN
TRACK PHASE - SWITCH CLOSED
(R2 + 1 k )
1k
R1
CHARGE
1k
R1
3pF
C1
) is given by the
3 pF
C1
3pF
V
BALANCE
IN
pin.
IN
pin
–11–
ADC TRANSFER FUNCTION
The output coding of the AD7816, AD7817 and AD7818 is
straight binary . The designed code transitions occur at succes-
sive integer LSB values (i.e., 1 LSB, 2 LSBs, etc.). The LSB
size is = +2.5 V/1024 = 2.44 mV. The ideal transfer characteris-
tic shown in Figure 10 below.
TEMPERATURE MEASUREMENT
The on-chip temperature sensor can be accessed via multiplexer
Channel 0, i.e., by writing 0 0 0 to the Channel Address Regis-
ter. The temperature is also the power on default selection. The
transfer characteristic of the temperature sensor is shown in
Figure 11 below. The result of the 10-bit conversion on Chan-
nel 0 can be converted to degrees centigrade by using the fol-
lowing equation.
Figure 11. Temperature Sensor Transfer Characteristic
000...010
000...000
000...001
111...111
111...110
111...000
011...111
1.2V
125°C
–55°C
Figure 10. ADC Transfer Function
0V
T
192Dec
Figure 9. On-Chip Reference
AMB
1LSB
= –103 C + (ADC Code/4)
AD7816/AD7817/AD7818
SW1
REF
ADC CODE
ANALOG INPUT
26k
IN
24k
1.2V
1LSB=2.5/1024
2.44mV
2.5V
912Dec
+2.5V•1LSB
EXTERNAL
REFERENCE
DETECT
BUFFER