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AD7278BRMZ-REEL Datasheet(PDF) 17 Page - Analog Devices

Part No. AD7278BRMZ-REEL
Description  3 MSPS, 12-/10-/8-Bit ADCs in 6-Lead TSOT
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD7278BRMZ-REEL Datasheet(HTML) 17 Page - Analog Devices

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AD7276/AD7277/AD7278
Rev. C | Page 17 of 28
The load regulation of the REF193 is typically 10 ppm/mA
(REF193, VS = 5 V), which results in an error of 50 ppm (150 μV)
for the 5 mA drawn from it. When VDD = 3 V from the REF193, it
corresponds to an error of 0.204 LSB, 0.051 LSB, and 0.0128 LSB
for the AD7276, AD7277, and AD7278, respectively. For applica-
tions where power consumption is of concern, use the power-down
mode of the ADC and the sleep mode of the REF19x reference to
improve power performance. See the Modes of Operation section.
AD7276/
AD7277/
AD7278
VDD
VIN
SERIAL
INTERFACE
0VTO VDD
INPUT
DSP/
µC/µP
GND
SCLK
CS
SDATA
0.1µF
10µF
1µF
TANT
0.1µF
680nF
3V
5V
SUPPLY
REF193
Figure 22. REF193 as Power Supply to the AD7276/AD7277/AD7278
Table 8 provides typical performance data with various
references used as a VDD source with the same setup conditions.
Table 8. AD7276 Performance (Various Voltage References IC)
Reference Tied to VDD
SNR Performance, 1 MHz Input
AD780 @ 3 V
71.3 dB
AD780 @ 2.5 V
70.1 dB
REF193
70.9 dB
Analog Input
Figure 23 shows an equivalent circuit of the analog input structure
of the AD7276/AD7277/AD7278. The two diodes, D1 and D2,
provide ESD protection for the analog inputs. Care must be taken
to ensure that the analog input signal never exceeds the supply
rails by more than 300 mV. Signals exceeding this value cause
these diodes to become forward biased and to start conducting
current into the substrate. These diodes can conduct a maximum
current of 10 mA without causing irreversible damage to the
part. Capacitor C1 in Figure 23 is typically about 4 pF and can
primarily be attributed to pin capacitance. Resistor R1 is a
lumped component made up of the on resistance of a switch.
This resistor is typically about 75 Ω. Capacitor C2 is the ADC
sampling capacitor and has a capacitance of 4 pF typically when
in hold mode and 32 pF typically when in track mode. For ac
applications, removing high frequency components from the
analog input signal is recommended by using a band-pass filter
on the relevant analog input pin. In applications where the
harmonic distortion and signal-to-noise ratio are critical, the
analog input should be driven from a low impedance source.
Large source impedances significantly affect the ac performance
of these ADCs and can necessitate the use of an input buffer
amplifier. The AD8021 op amp is compatible with these devices;
however, the choice of the op amp is a function of the particular
application.
C1
4pF
C2
R1
CONVERSION PHASE—SWITCH OPEN
TRACK PHASE—SWITCH CLOSED
D1
D2
VDD
VIN
Figure 23. Equivalent Analog Input Circuit
When no amplifier is used to drive the analog input, the source
impedance should be limited to a low value. The maximum source
impedance depends on the amount of THD that can be tolerated.
The THD increases as the source impedance increases and per-
formance degrades. Figure 16 shows a graph of the THD vs. the
analog input frequency for different source impedances when
using a supply voltage of 3 V and sampling at a rate of 3 MSPS.
Digital Inputs
The digital inputs applied to the AD7276/AD7277/AD7278 are
not limited by the maximum ratings that limit the analog inputs.
Instead, the digital inputs applied to the AD7276/AD7277/
AD7278 can be 6 V and are not restricted by the VDD + 0.3 V
limit of the analog inputs. For example, if the AD7276/AD7277/
AD7278 are operated with a VDD of 3 V, then 5 V logic levels can
be used on the digital inputs. However, it is important to note
that the data output on SDATA still has 3 V logic levels when
VDD = 3 V. Another advantage of SCLK and CS not being restricted
by the VDD + 0.3 V limit is that power supply sequencing issues are
avoided. For example, unlike with the analog inputs, with the
digital inputs, if CS or SCLK is applied before VDD, there is no
risk of latch-up.


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