AD7890BN-4 Analog Devices Inc, AD7890BN-4 Datasheet - Page 11

• Manufacturer Part Number: AD7890BN-4
• Description: IC DAS 12BIT 8CH 24-DIP
• Manufacturer: Analog Devices Inc
• Type: Data Acquisition System (DAS)r

Specifications of AD7890BN-4

• Rohs Status: RoHS non-compliant
• Resolution (bits): 12 b
• Sampling Rate (per Second): 117k
• Data Interface: Serial
• Voltage Supply Source: Single Supply
• Voltage - Supply: 0 V ~ 4.10 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Through Hole
• Package / Case: 24-DIP (0.300", 7.62mm)

THEORY OF OPERATION
CONVERTER DETAILS
The AD7890 is an 8-channel, 12-bit, single supply, serial data
acquisition system. It provides the user with signal scaling,
multiplexer, track/hold, reference, ADC, and versatile serial
logic functions on a single chip. The signal scaling allows the
part to handle ±10 V input signals (AD7890-10) and 0 V to
4.096 V input signals (AD7890-4) while operating from a single
5 V supply. The AD7890-2 contains no signal scaling and
accepts an analog input range of 0 V to 2.5 V. The part operates
from a 2.5 V reference, which can be provided from the part’s
own internal reference or from an external reference source.
Unlike other single chip data acquisition solutions, the AD7890
provides the user with separate access to the multiplexer and
the ADC. This means that the flexibility of separate multiplexer
and ADC solutions is not sacrificed with the one-chip solution.
With access to the multiplexer output, the user can implement
external signal conditioning between the multiplexer and the
track/hold. It means that one antialiasing filter can be used on
the output of the multiplexer to provide the antialiasing
function for all eight channels.
Conversion is initiated on the AD7890 either by pulsing the
CONVST input or by writing a Logic 1 to the CONV bit of the
control register. When using the hardware CONVST input, on
the rising edge of the CONVST signal, the on-chip track/hold
goes from track to hold mode and the conversion sequence is
started, provided the internal pulse has timed out. This internal
pulse (which appears at the C
multiplexer address is loaded to the AD7890 control register.
This pulse goes from high to low when a serial write to the part
is initiated. It starts to discharge on the sixth falling clock edge
of SCLK in a serial write operation to the part. The track/hold
cannot go into hold and conversion cannot be initiated until the
C
time of the voltage on C
connected to the C
The fact that the pulse is initiated every time a write to the
control register takes place means that the software conversion
start and track/hold signal is always delayed by the internal pulse.
The conversion clock for the part is generated from the clock
signal applied to the CLK IN pin of the part. Conversion time
for the AD7890 is 5.9 μs from the rising edge of the hardware
CONVST signal and the track/hold acquisition time is 2 μs. To
obtain optimum performance from the part, the data read
operation or control register write operation should not occur
during the conversion or during 500 ns prior to the next
conversion.
EXT
pin has crossed its trigger point of 2.5 V. The discharge
EXT
pin (see the C
EXT
depends upon the value of capacitor
EXT
pin) is initiated whenever the
EXT
Functioning section).
Rev. C | Page 11 of 28
This allows the part to operate at throughput rates up to
117 kHz in the external clocking mode and achieve data sheet
specifications. The part can operate at slightly higher
throughput rates (up to 127 kHz), again in external clocking
mode with degraded performance (see the Timing and Control
section). The throughput rate for self-clocking mode is limited
by the serial clock rate to 78 kHz.
All unused inputs should be connected to a voltage within the
nominal analog input range to avoid noise pickup. On the
AD7890-10, if any one of the input channels which are not
being converted goes more negative than −12 V, it can interfere
with the conversion on the selected channel.
CIRCUIT DESCRIPTION
The AD7890 is offered as three part types: the AD7890-10
handles a ±10 V input voltage range, the AD7890-4 handles a
0 V to 4.096 V input range, while the AD7890-2 handles a 0 V
to 2.5 V input voltage range.
AD7890-10 Analog Input
Figure 4 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 (such as:
1 LSB, 2 LSBs, 3 LSBs...). Output coding is twos complement
binary with 1 LSB − FSR/4096 = 20 V/4096 = 4.88 mV. The
ideal input/output transfer function is shown in Table 4.
REF OUT/
REF IN
AGND
V
INX
Figure 4. AD7890-10 Analog Input Structure
1
EQUIVALENT ON-RESISTANCE OF MULTIPLEXER
2kΩ
30kΩ
REFERENCE
2.5V
7.5kΩ
10kΩ
REFERENCE
CIRCUITRY
AD7890-10
TO ADC
200Ω
1
MUX OUT
AD7890