AD7671 Analog Devices, AD7671 Datasheet - Page 14

• Manufacturer Part Number: AD7671
• Manufacturer: Analog Devices

Specifications of AD7671

• Resolution (bits): 16bit
• # Chan: 1
• Sample Rate: 1MSPS
• Interface: Par,Ser,SPI
• Analog Input Type: Diff-Bip,Diff-Uni
• Ain Range: Bip (Vref),Bip (Vref) x 2,Bip (Vref) x 4,Uni (Vref),Uni (Vref) x 2,Uni (Vref) x 4
• Adc Architecture: SAR
• Pkg Type: CSP,QFP

AD7671
Analog Inputs
The AD7671 is specified to operate with six full-scale analog input
ranges. Connections required for each of the four analog inputs,
IND, INC, INB, and INA, and the resulting full-scale ranges
are shown in Table I. The typical input impedance for each
analog input range is also shown.
Figure 6 shows a simplified analog input section of the AD7671.
The four resistors connected to the four analog inputs form a
resistive scaler that scales down and shifts the analog input range
to a common input range of 0 V to 2.5 V at the input of the
switched capacitive ADC.
By connecting the four inputs INA, INB, INC, and IND to the
input signal itself, the ground, or a 2.5 V reference, other analog
input ranges can be obtained.
The diodes shown in Figure 6 provide ESD protection for the
four analog inputs. The inputs INB, INC, and IND have a high
voltage protection (–11 V to +30 V) to allow a wide input voltage
range. Care must be taken to ensure that the analog input signal
never exceeds the absolute ratings on these inputs, including
INA (0 V to 5 V). This will cause these diodes to become forward-
biased and start conducting current. These diodes can handle
a forward-biased current of 120 mA maximum. For instance,
when using the 0 V to 2.5 V input range, these conditions could
eventually occur on the input INA when the input buffer’s (U1)
supplies are different from AVDD. In such cases, an input buffer
with a short-circuit current limitation can be used to protect the part.
This analog input structure allows the sampling of the differen-
tial signal between the output of the resistive scaler and INGND.
Unlike other converters, the INGND input is sampled at the
same time as the inputs. By using this differential input, small
signals common to both inputs are rejected as shown in Figure 7,
which represents the typical CMRR over frequency. For instance,
by using INGND to sense a remote signal ground, the difference of
ground potentials between the sensor and the local ADC ground
is eliminated. During the acquisition phase for ac signals, the
AD7671 behaves like a one-pole RC filter consisting of the
equivalent resistance of the resistive scaler R/2 in series with R1
and C
component made up of some serial resistors and the on resis-
tance of the switches.
The capacitor C
sampling capacitor. This one-pole filter with a typical –3 dB
cutoff frequency of 9.6 MHz reduces undesirable aliasing effects
and limits the noise coming from the inputs.
S
. The resistor R1 is typically 100 W and is a lumped
IND
INC
INB
INA
Figure 6. Simplified Analog Input
S
is typically 60 pF and is mainly the ADC
AGND
AVDD
R = 1.28k
4R
4R
2R
R
R1
C
S
–14–
Except when using the 0 V to 2.5 V analog input voltage range,
the AD7671 has to be driven by a very low impedance source to
avoid gain errors. That can be done by using a driver amplifier
whose choice is eased by the primarily resistive analog input
circuitry of the AD7671.
When using the 0 V to 2.5 V analog input voltage range, the input
impedance of the AD7671 is very high so the AD7671 can be
driven directly by a low impedance source without gain error.
That allows, as shown in Figure 5, putting an external one-pole
RC filter between the output of the amplifier output and the
ADC analog inputs to even further improve the noise filtering
done by the AD7671 analog input circuit. However, the source
impedance has to be kept low because it affects the ac perfor-
mances, especially the total harmonic distortion (THD). The
maximum source impedance depends on the amount of total THD
that can be tolerated. The THD degradation is a function of the
source impedance and the maximum input frequency as shown
in Figure 8.
Figure 8. THD vs. Analog Input Frequency and Input
Resistance (0 V to 2.5 V Only)
–100
–110
–70
–80
–90
Figure 7. Analog Input CMRR vs. Frequency
75
70
65
60
55
50
45
40
35
1
0
10
R = 50
FREQUENCY – kHz
FREQUENCY – kHz
100
100
R = 100
1000
R = 11
10000
1000
REV. B