AD7715ANZ-3 Analog Devices Inc, AD7715ANZ-3 Datasheet - Page 15

• Manufacturer Part Number: AD7715ANZ-3
• Description: IC ADC 16BIT SIGMA-DELTA 16DIP
• Manufacturer: Analog Devices Inc

Specifications of AD7715ANZ-3

• Data Interface: DSP, MICROWIRE™, QSPI™, Serial, SPI™
• Number Of Bits: 16
• Sampling Rate (per Second): 500
• Number Of Converters: 1
• Power Dissipation (max): 9.5mW
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Through Hole
• Package / Case: 16-DIP (0.300", 7.62mm)
• Resolution (bits): 16bit
• Input Channel Type: Differential
• Supply Voltage Range - Analogue: 3V To 3.6V
• Supply Voltage Range - Digital: 3V To 5.25V
• Supply Current: 600µA
• No. Of: RoHS Compliant
• Sampling Rate: 19.2kSPS
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With: EVAL-AD7715-3EBZ - BOARD EVALUATION FOR AD7715
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

REV. C
ANALOG INPUT
Analog Input Ranges
The AD7715 contains a differential analog input pair AIN(+)
and AIN(–). This input pair provides a programmable-gain,
differential input channel which can handle either unipolar or
bipolar input signals. It should be noted that the bipolar input
signals are referenced to the respective AIN(–) input of the
input pair.
In unbuffered mode, the common-mode range of the input is
from AGND to AV
analog input voltage lies between AGND – 30 mV and
AV
can handle both unipolar and bipolar input ranges for all gains.
In buffered mode, the analog inputs can handle much larger
source impedances but the absolute input voltage range is re-
stricted to between AGND + 50 mV to AV
also places restrictions on the common-mode range. This means
that in buffered mode there are some restrictions on the allow-
able gains for bipolar input ranges. Care must be taken in set-
ting up the common-mode voltage and input voltage range so
that the above limits are not exceeded, otherwise there will be a
degradation in linearity performance.
In unbuffered mode, the analog inputs look directly into the
input sampling capacitor, C
in this unbuffered mode is 1 nA maximum. As a result, the
analog inputs see a dynamic load that is switched at the input
sample rate (see Figure 3). This sample rate depends on master
clock frequency and selected gain. C
and discharged to AIN(–) every input sample cycle. The effec-
tive on-resistance of the switch, R
+5V SUPPLY
DD
ANALOG
+5V SUPPLY
ANALOG
+ 30 mV. This means that in unbuffered mode the part
Figure 2. AD7715-5 Basic Connection Diagram
Figure 3. Unbuffered Analog Input Structure
AIN(+)
AIN(–)
AD780
GND
V
V
IN
OUT
ANALOG INPUT
DIFFERENTIAL
SWITCHING FREQUENCY
DEPENDS ON
AND SELECTED GAIN
GROUND
GROUND
ANALOG
DIGITAL
10 F
DD
10 F
provided that the absolute value of the
0.1 F
0.1 F
R
f
SAMP
CLKIN
SW
(7k
. The dc input leakage current
(10pF )
AIN(+)
AIN(–)
AGND
DGND
REF IN(+)
REF IN(–)
SW
C
AD7715
SAMP
TYP)
AV
, is typically 7 k .
SAMP
DD
MCLK OUT
V
MCLK IN
BIAS
RESET
DV
DRDY
DOUT
SCLK
is charged to AIN(+)
DD
DIN
CS
DD
IMPEDANCE
– 1.5 V which
HIGH
1G
+5V
CRYSTAL OR
CERAMIC
RESONATOR
0.1 F
SERIAL DATA
DATA READY
RECEIVE (READ)
SERIAL CLOCK
–15–
C
source impedances every input sample cycle. Therefore, in
unbuffered mode, source impedances mean a longer charge time
for C
XIV shows the allowable external resistance/capacitance values,
for unbuffered mode, such that no gain error to the 16-bit level
is introduced on the part. Note that these capacitances are total
capacitances on the analog input, external capacitance plus
10 pF capacitance from the pins and lead frame of the device.
Gain
1
2
32
128
In buffered mode, the analog inputs look into the high imped-
ance inputs stage of the on-chip buffer amplifier. C
charged via this buffer amplifier such that source impedances do
not affect the charging of C
offset leakage current of 1 nA. In this buffered mode, large
source impedances result in a small dc offset voltage developed
across the source impedance but not in a gain error.
Input Sample Rate
The modulator sample frequency for the AD7715 remains at
f
the selected gain. However, gains greater than 1 are achieved by
a combination of multiple input samples per modulator cycle
and a scaling of the ratio of reference capacitor to input capaci-
tor. As a result of the multiple sampling, the input sample rate
of the device varies with the selected gain (see Table XV). In
buffered mode, the input is buffered before the input sampling
Gain
1
2
32
128
capacitor. In unbuffered mode, where the analog input looks
directly into the sampling capacitor, the effective input imped-
ance is 1/C
tance and f
Bipolar/Unipolar Inputs
The analog input on the AD7715 can accept either unipolar or
bipolar input voltage ranges. Bipolar input ranges do not imply
that the part can handle negative voltages on its analog input
since the analog input cannot go more negative than –30 mV to
ensure correct operation of the part. The input channel is fully
differential. As a result, the voltage to which the unipolar and
bipolar signals on the AIN(+) input are referenced is the voltage
on the respective AIN(–) input. For example, if AIN(–) is
+2.5 V and the AD7715 is configured for unipolar operation
CLK IN
SAMP
Table XIV. External R, C Combination for No 16-Bit Gain
Error (Unbuffered Mode Only)
SAMP
/128 (19.2 kHz @ f
must be charged through R
10
152 k
75.1 k
16.7 k
16.7 k
Table XV. Input Sampling Frequency vs. Gain
, and this may result in gain errors on the part. Table
SAMP
S
f
2
8
8
Input Sampling Freq (f
CLK IN
is the input sample rate.
f
f
f
CLK IN
CLK IN
CLK IN
50
53.9 k
26.6 k
5.95 k
5.95 k
External Capacitance (pF)
f
/64 (38.4 kHz @ f
S
where C
/64 (76.8 kHz @ f
/64 (307.2 kHz @ f
/64 (307.2 kHz @ f
CLK IN
SAMP
100
31.4 k
15.4 k
3.46 k
3.46 k
SAMP
. This buffer amplifier has an
= 2.4576 MHz) regardless of
is the input sampling capaci-
SW
CLK IN
and through any external
500
8.4 k
4.14 k
924
924
S
)
CLK IN
CLK IN
CLK IN
= 2.4576 MHz)
4.76 k
2.36 k
526
526
1000
= 2.4576 MHz)
AD7715
= 2.4576 MHz)
= 2.4576 MHz)
SAMP
is
5000
1.36 k
670
150
150