AD7890
CIRCUIT DESCRIPTION
Analog Input Section
The AD7890 is offered as three part types, the AD7890-10
which handles a
±
10 V input voltage range, the AD7890-4
which handles a 0 V to +4.096 V input range and the AD7890-2
which handles a 0 V to +2.5 V input voltage range.
AD7890-10
Figure 2 shows the analog input section for the AD7890-10.
The analog input range for each of the analog inputs is
±
10 V
into an input resistance of typically 33 kΩ. This input is benign
with no dynamic charging currents with the resistor attenuator
stage followed by the multiplexer and in cases where MUX
OUT is connected to SHA IN this is followed by the high input
impedance stage of the track/hold amplifier. The designed code
transitions occur on successive integer LSB values (i.e., 1 LSB,
2 LSBs, 3 LSBs...). Output coding is 2s complement binary
with 1 LSB – FS/4096 = 20 V/4096 = 4.88 mV. The ideal input/
output transfer function is shown in Table I.
MUX OUT
+2.5V
REFERENCE
tions occur on successive integer LSB values (i.e., 1 LSB,
2 LSBs, 3 LSBs . . . ). Output coding is straight (natural)
binary with 1 LSB = FS/4096 = 4.096 V/4096 = 1 mV. The
ideal input/output transfer function is shown in Table II.
MUX OUT
+2.5V
REFERENCE
2kΩ
REF OUT/
REF IN
TO ADC
REFERENCE
CIRCUITRY
6kΩ
V
INX
9.38kΩ
AGND
200Ω*
AD7890-4
*EQUIVALENT ON-RESISTANCE OF MULTIPLEXER
Figure 3. AD7890-4 Analog Input Structure
Table II. Ideal Input/Output Code Table for the AD7890-4
TO ADC
REFERENCE
CIRCUITRY
2kΩ
REF OUT/
REF IN
Analog Input
1
Digital Output
Code Transition
111 . . . 110 to 111 . . . 111
111 . . . 101 to 111 . . . 110
111 . . . 100 to 111 . . . 101
000 . . . 010 to 000 . . . 011
000 . . . 001 to 000 . . . 010
000 . . . 000 to 000 . . . 001
7.5kΩ
30kΩ
V
INX
10kΩ
AGND
200Ω*
+FSR – 1 LSB
2
(4.095 V)
+FSR – 2 LSBs (4.094 V)
+FSR – 3 LSBs (4.093 V)
AGND + 3 LSBs (0.003 V)
AGND + 2 LSBs (0.002 V)
AGND + 1 LSB (0.001 V)
AD7890-10
*EQUIVALENT ON-RESISTANCE OF MULTIPLEXER
Figure 2. AD7890-10 Analog Input Structure
NOTES
1
FSR is full-scale range and is 4.096 V with REF IN +2.5 V.
2
1 LSB = FSR/4096 = 1 mV with REF IN = +2.5 V.
Table I. Ideal Input/Output Code Table for the AD7890-10
Analog Input
1
Digital Output
Code Transition
011 . . . 110 to 011 . . . 111
011 . . . 101 to 011 . . . 110
011 . . . 100 to 011 . . . 101
000 . . . 000 to 000 . . . 001
111 . . . 111 to 000 . . . 000
111 . . . 110 to 111 . . . 111
100 . . . 010 to 100 . . . 011
100 . . . 001 to 100 . . . 010
100 . . . 000 to 100 . . . 001
+FSR/2 – 1 LSB
2
(9.995117 V)
+FSR/2 – 2 LSBs (9.990234 V)
+FSR/2 – 3 LSBs (9.985352 V)
AGND + 1 LSB (0.004883 V)
AGND (0.000000 V)
AGND – 1 LSB (–0.004883 V)
–FSR/2 + 3 LSBs (–9.985352 V)
–FSR/2 + 2 LSBs (–9.990234 V)
–FSR/2 + 1 LSB (–9.995117 V)
AD7890-2
The analog input section for the AD7890-2 contains no biasing
resistors and the selected analog input connects to the multi-
plexer and in cases where MUX OUT is connected to SHA IN
this is followed by the high input impedance stage of the track/
hold amplifier. The analog input range is, therefore, 0 V to +2.5 V
into a high impedance stage with an input current of less than
50 nA. The designed code transitions occur on successive integer
LSB values (i.e., l LSB, 2 LSBs, 3 LSBs . . . FS-1 LSBs). Out-
put coding is straight (natural) binary with 1 LSB = FS/4096 =
2.5 V/4096 = 0.61 mV. The ideal input/output transfer function
is shown in Table III.
Table III. Ideal Input/Output Code Table for the AD7890-2
NOTES
1
FSR is full-scale range and is 20 V with REF IN = +2.5 V.
2
1 LSB = FSR/4096 = 4.883 mV with REF IN = +2.5 V.
Analog Input
1
Digital Output
Code Transition
111 . . . 110 to 111 . . . 111
111 . . . 101 to 111 . . . 110
111 . . . 100 to 111 . . . 101
000 . . . 010 to 010 . . . 011
000 . . . 001 to 001 . . . 010
000 . . . 000 to 000 . . . 001
AD7890-4
Figure 3 shows the analog input section for the AD7890-4. The
analog input range for each of the analog inputs is
±
10 V into
an input resistance of typically 15 kΩ. This input is benign with
no dynamic charging currents with the resistor attenuator stage
followed by the multiplexer and in cases where MUX OUT is
connected to SHA IN this is followed by the high input imped-
ance stage of the track/hold amplifier. The designed code transi-
+FSR – 1 LSB
2
(2.499390 V)
+FSR – 2 LSBs (2.498779 V)
+FSR – 3 LSBs (2.498169 V)
AGND + 3 LSBs (0.001831 V)
AGND + 2 LSBs (0.001221 V)
AGND + 1 LSB (0.000610 V)
NOTES
1
FSR is full-scale range and is 2.5 V with REF IN = +2.5 V.
2
1 LSB = FSR/4096 = 0.61 mV with REF IN = +2.5 V.
REV. A
–9–
AD [ ANALOG DEVICES ]
