AD7890
PIN FUNCTION DESCRIPTION
Pin
1
2
Mnemonic
AGND
SMODE
Description
Analog Ground. Ground reference for track/hold, comparator and DAC.
Control Input. Determines whether the part operates in its External Clocking (slave) or Self-Clocking
(master) serial mode. With SMODE at a logic low, the part is in its Self-Clocking serial mode with
RFS
and SCLK as outputs. This Self-Clocking mode is useful for connection to shift registers or to
serial ports of DSP processors. With SMODE at a logic high, the part is in its External Clocking
serial mode with SCLK and
RFS
as inputs. This External Clocking mode is useful for connection to
the serial port of microcontrollers such as the 8XC51 and the 68HCXX and for connection to the
serial ports of DSP processors.
Digital Ground. Ground reference for digital circuitry.
External Capacitor. An external capacitor is connected to this pin to determine the length of the
internal pulse (see
CONVST
input and Control Register section). Larger capacitances on this pin
extend the pulse to allow for settling time delays through an external antialiasing filter or signal
conditioning circuitry.
Convert Start. Edge-triggered logic input. A low to high transition on this input puts the track/hold
into hold and initiates conversion provided that the internal pulse has timed out (see Control
Register section). If the internal pulse is active when the
CONVST
goes high, the track/hold will not
go into hold until the pulse times out. If the internal pulse has timed out when
CONVST goes
high,
the rising edge of
CONVST
drives the track/hold into hold and initiates conversion.
Clock Input. An external TTL-compatible clock is applied to this input pin to provide the clock
source for the conversion sequence. In the Self-Clocking serial mode, the SCLK output is derived
from this CLK IN pin.
Serial Clock Input. In the External Clocking (slave) mode (see Serial Interface section) this is an
externally applied serial clock which is used to load serial data to the control register and to access
data from the output register. In the Self-Clocking (master) mode, the internal serial clock, which is
derived from the clock input (CLK IN), appears on this pin. Once again, it is used to load serial data
to the control register and to access data from the output register.
Transmit Frame Synchronization Pulse. Active low logic input with serial data expected after the
falling edge of this signal.
Receive Frame Synchronization Pulse. In the External Clocking mode, this pin is an active low logic
input with
RFS
provided externally as a strobe or framing pulse to access serial data from the output
register. In the Self-Clocking mode, it is an active low output which is internally generated and
provides a strobe or framing pulse for serial data from the output register. For applications which
require that data be transmitted and received at the same time,
RFS
and
TFS
should be connected
together.
Serial Data Output. Sixteen bits of serial data are provided with one leading zero, preceding the three
address bits of the Control register and the 12 bits of conversion data. Serial data is valid on the
falling edge of SCLK for sixteen edges after
RFS
goes low. Output coding from the ADC is 2s
complement for the AD7890-10 and straight binary for the AD7890-4 and AD7890-2.
Serial Data Input. Serial data to be loaded to the control register is provided at this input. The first
five bits of serial data are loaded to the control register on the first five falling edges of SCLK after
TFS
goes low. Serial data on subsequent SCLK edges is ignored while
TFS
remains low.
Positive supply voltage, +5 V
±
5%.
Multiplexer Output. The output of the multiplexer appears at this pin. The output voltage range
from this output is 0 V to +2.5 V for the nominal analog input range to the selected channel. The
output impedance of this output is nominally 3.5 kΩ. If no external antialiasing filter is required,
MUX OUT should be connected to SHA IN.
Track/Hold Input. The input to the on-chip track/hold is applied to this pin. It is a high impedance
input and the input voltage range is 0 V to +2.5 V.
Analog Ground. Ground reference for track/hold, comparator and DAC.
Analog Input Channel 1. Single-ended analog input. The analog input range on is
±
10 V
(AD7890-10), 0 V to +4.096 V (AD7890-4) and 0 V to +2.5 V (AD7890-2). The channel to be con-
verted is selected using the A0, A1 and A2 bits in the control register. The multiplexer has guaran-
teed break-before-make operation.
–5–
3
4
DGND
C
EXT
5
CONVST
6
CLK IN
7
SCLK
8
9
TFS
RFS
10
DATA OUT
11
DATA IN
12
13
V
DD
MUX OUT
14
15
16
SHA IN
AGND
V
IN1
REV. A
AD [ ANALOG DEVICES ]
