AD7887AR-REEL7 Analog Devices Inc, AD7887AR-REEL7 Datasheet - Page 12

• Manufacturer Part Number: AD7887AR-REEL7
• Description: IC,Data Acquisition System,2-CHANNEL,12-BIT,SOP,8PIN,PLASTIC
• Manufacturer: Analog Devices Inc

Specifications of AD7887AR-REEL7

• Rohs Status: RoHS non-compliant
• Design Resources: Software Calibrated, 1 MHz to 8 GHz, 70 dB RF Power Measurement System Using AD8318 (CN0150)
• Number Of Bits: 12
• Sampling Rate (per Second): 125k
• Data Interface: DSP, MICROWIRE™, QSPI™, Serial, SPI™
• Number Of Converters: 1
• Power Dissipation (max): 3.5mW
• Voltage Supply Source: Single Supply
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (0.154", 3.90mm Width)

AD7887
ANALOG INPUT
Figure 12 shows an equivalent circuit of the analog input
structure of the AD7887. 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 exceed the supply rails
by more than 200 mV. Exceeding this value causes the diodes
to become forward biased and to start conducting into the
substrate. The maximum current these diodes can conduct
without causing irreversible damage to the part is 20 mA.
However, it is worth noting that a small amount of current
(1 mA) being conducted into the substrate due to an
overvoltage on an unselected channel can cause inaccurate
conversions on a selected channel. Capacitor C1 in Figure 12 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 multiplexer and a switch. This resistor is
typically about 100 Ω. Capacitor C2 is the ADC sampling
capacitor and typically has a capacitance of 20 pF.
Note that the analog input capacitance seen when in track mode
is typically 38 pF, whereas in hold mode it is typically 4 pF.
For ac applications, removing high frequency components from
the analog input signal is recommended by use of an RC
low-pass filter on the relevant analog input pin. In applications
where harmonic distortion and signal-to-noise ratio are critical,
the analog input should be driven from a low impedance
source. Large source impedances will significantly affect the ac
performance of the ADC. This may necessitate the use of an
input buffer amplifier. The choice of op amp is a function of the
particular application.
When no amplifier is used to drive the analog input, the source
impedance should be limited to low values. The maximum
source impedance depends on the amount of total harmonic
distortion (THD) that can be tolerated. The THD increases as
the source impedance increases and performance degrades.
Figure 13 shows a graph of the total harmonic distortion vs. the
analog input signal frequency for different source impedances.
V
IN
C1
4pF
Figure 12. Equivalent Analog Input Circuit
V
DD
D1
D2
CONVERSION PHASE—SWITCH OPEN
TRACK PHASE—SWITCH CLOSED
R1
20pF
C2
Rev. D | Page 12 of 24
On-Chip Reference
The AD7887 has an on-chip 2.5 V reference. This reference can
be enabled or disabled by clearing or setting the REF bit in the
control register, respectively. If the on-chip reference is to be used
externally in a system, it must be buffered before it is applied
elsewhere. If an external reference is applied to the device, the
internal reference is automatically overdriven. However, it is
advised to disable the internal reference by setting the REF bit
in the control register when an external reference is applied in
order to obtain optimum performance from the device. When
the internal reference is disabled, SW1, shown in Figure 14,
opens and the input impedance seen at the AIN1/V
input impedance of the reference buffer, which is in the region
of gigaohms. When the internal reference is enabled, the input
impedance seen at the pin is typically 10 kΩ. When the AD7887
is operated in two-channel mode, the reference is taken from
V
DD
internally, not from the on-chip 2.5 V reference.
–65
–70
–75
–80
–85
–90
0
1 .
5
THD vs. FREQUENCY FOR DIFFERENT
SOURCE IMPEDANCES
AIN1/V
V
5V EXT REFERENCE
DD
= 5V
Figure 13. THD vs. Analog Input Frequency
REF
Figure 14. On-Chip Reference Circuitry
10.89
R
2.5V
INPUT FREQUENCY (kHz)
IN
SW1
= 10Ω, C
21.14
10kΩ
R
IN
IN
= 10nF
= 50Ω, C
31.59
R
IN
IN
= 1kΩ, C
= 2.2nF
4
IN
2
1 .
= 100pF
4
REF
pin is the
49.86