Datasheet AD7899 (Analog Devices) - 10

AD7899 AD7899-2 AD7899-1
Figure 3 shows the analog input section of the AD7899-2. Each
2.5V
input can be configured for 0 V to 5 V operation or 0 V to 2.5 V
REFERENCE
operation. For 0 V to 5 V operation, the VINB input is tied to GND and the input voltage is applied to the V
6k
⍀ INA input. For 0 V to 2.5 V operation, the VINA and VINB inputs are tied together
TO ADC
and the input voltage is applied to both. The V
REFERENCE
INA and VINB
VREF CIRCUITRY
inputs are symmetrical and fully interchangeable.
R1
For the AD7899-2, R1 = 4 kΩ and R2 = 4 kΩ. Once again, the
R2 V TO INTERNAL
designed code transitions occur on successive integer LSB values.
INA COMPARATOR
Output coding is straight (natural) binary with 1 LSB = FSR/
TRACK/HOLD
16384 = 2.5 V/16384 = 0.153 mV, and 5 V/16384 = 0.305 mV,
R3
for the 0 to 2.5 V and the 0 to 5 V options respectively. Table
VINB
II shows the ideal input and output transfer function for the
R4
AD7899-2.
GND AD7899-2
Figure 2. AD7899-1 Analog Input Structure
2.5V
For the AD7899-1, R1 = 4 kΩ, R2 = 16 kΩ, R3 = 16 kΩ and
REFERENCE
R4 = 8 kΩ. The resistor input stage is followed by the high
6k
⍀ input impedance stage of the track/hold amplifier. The designed code transitions take place midway between suc-
TO ADC REFERENCE
cessive integer LSB values (i.e., 1/2 LSB, 3/2 LSBs, 5/2 LSBs
VREF CIRCUITRY
etc.) LSB size is given by the formula, 1 LSB = FSR/16384. For the ±5 V range, 1 LSB = 10 V/16384 = 610.4 µV. For the ±10 V
R1 V TO INTERNAL
range, 1 LSB = 20 V/16384 = 1.22 mV. Output coding is
INA COMPARATOR TRACK/HOLD
two’s complement binary with 1 LSB = FSR/16384. The ideal input/output transfer function for the AD7899-1 is shown in
R2
Table I.
VINB Table I. Ideal Input/Output Code Table for the AD7899-1 Digital Output
Figure 3. AD7899-2 Analog Input Structure
Analog Input1 Code Transition Table II. Ideal Input/Output Code Table for the AD7899-2
+FSR/2 – 3/2 LSB2 011 . 110 to 011 . 111 +FSR/2 – 5/2 LSB 011 . 101 to 011 . 110
Digital Output
+FSR/2 – 7/2 LSB 011 . 100 to 011 . 101
Analog Input1 Code Transition
GND + 3/2 LSB 000 . 001 to 000 . 010 +FSR – 3/2 LSB2 111 . 110 to 111 . 111 GND + 1/2 LSB 000 . 000 to 000 . 001 +FSR – 5/2 LSB 111 . 101 to 111 . 110 GND – 1/2 LSB 111 . 111 to 000 . 000 +FSR – 7/2 LSB 111 . 100 to 111 . 101 GND – 3/2 LSB 111 . 110 to 111 . 111 GND + 5/2 LSB 000 . 010 to 000 . 011 –FSR/2 + 5/2 LSB 100 . 010 to 100 . 011 GND + 3/2 LSB 000 . 001 to 000 . 010 –FSR/2 + 3/2 LSB 100 . 001 to 100 . 010 GND + 1/2 LSB 000 . 000 to 000 . 001 –FSR/2 + 1/2 LSB 100 . 000 to 100 . 001 NOTES 1FSR is Full-Scale Range and is 0 to 2.5 V and 0 to 5 V for AD7899-2 with V NOTES REF 1 = 2.5 V. FSR is full-scale range and is 20 V for the ±10 V range and 10 V for the ±5 V 21 LSB = FSR/16384 and is 0.153 mV (0 to 2.5 V) and 0.305 mV (0 to 5 V) for range, with VREF = 2.5 V. 2 AD7899-2 with V 1 LSB = FSR/16384 = 1.22 mV (±10 V – AD7899-1) and 610.4 µV (±5 V – REF = 2.5 V. AD7899-1) with VREF = 2.5 V. REV. A –9–