
AD7484
Rev. C | Page 14 of 20
Figure 18 shows the AD7484 conversion sequence when the
part is put into nap mode after each conversion.
600ns
NAP
300ns
1400ns
2s
CONVST
BUSY
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018
Figure 18. Nap Mode Power Dissipation
of power vs. throughput for the AD7484 when in normal mode
and nap mode, respectively.
THROUGHPUT (kSPS)
60
0
3000
PO
W
ER
(m
W
)
500
1000
1500
2000
2500
65
70
75
80
85
90
02642-
019
Figure 19. Normal Mode, Power vs. Throughput
THROUGHPUT (kSPS)
0
2000
250
PO
W
ER
(m
W
)
750
1250
1500
1750
10
500
1000
20
30
40
50
60
70
80
90
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020
Figure 20. Nap Mode, Power vs. Throughput
In standby mode, all internal circuitry is powered down and the
power consumption of the AD7484 is reduced to 10 W. The
power-up time necessary before a conversion can be initiated is
longer because more of the internal circuitry has been powered
down. In using the internal reference of the AD7484, the ADC
must be brought out of standby mode 500 ms before a conversion is
initiated. Initiating a conversion before the required power-up time
has elapsed results in incorrect conversion data. If an external
reference source is used and kept powered up while the AD7484 is
in standby mode, the power-up time required is reduced to 80 s.
OFFSET/OVERRANGE
The AD7484 provides a ±8% overrange capability as well as a
programmable offset register. The overrange capability is achieved
by the use of a 15th bit (D14) and the CLIP input. If the CLIP input
is at logic high and the contents of the offset register are 0, then the
AD7484 operates as a normal 14-bit ADC. If the input voltage is
greater than the full-scale voltage, the data output from the ADC is
all 1s. Similarly, if the input voltage is lower than the zero-scale
voltage, the data output from the ADC is all 0s. In this case, D14
acts as an overrange indicator. It is set to 1 if the analog input
voltage is outside the nominal 0 V to 2.5 V range.
The default contents of the offset register are 0. If the offset reg-
ister contains any value other than 0, the contents of the register
are added to the SAR result at the end of conversion. This has the
effect of shifting the transfer function of the ADC as shown in
the CLIP input set to logic high, the maximum and minimum
codes that the AD7484 can output are 0x3FFF and 0x0000,
Figure 21 shows the effect of writing a positive value to the
offset register. For example, if the contents of the offset register
contained the value 1024, then the value of the analog input
voltage for which the ADC transitions from reading all 0s to
000…001 (the bottom reference point) is
0.5 LSB (1024 LSB) = 156.326 mV
The analog input voltage for which the ADC reads full-scale
(0x3FFF) in this example is
2.5 – 1.5 LSB – (1024 LSB) = 2.34352 V
ANALOG INPUT
0V
1LSB = VREF/16384
0.
5L
S
B
–
OFFS
E
T
000...000
ADC
CO
DE
111...111
000...001
000...010
111...110
111...000
011...111
+VREF – 1.5LSB
–OFFSET
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021
Figure 21. Transfer Characteristic with Positive Offset
The effect of writing a negative value to the offset register is
shown in
Figure 22. If a value of 512 is written to the offset
register, the bottom end reference point occurs at
0.5 LSB – (512 LSB) = 78.20 mW
Following this, the analog input voltage needed to produce a
full-scale (0x3FFF) result from the ADC is
2.5 V – 1.5 LSB – (512 LSB) = 2.5779 V