log input to pin 13 for the 0V to 10V range or to
pin 14 for the 0V to 20V range.
of these inputs in controlling the converter’s
operation is shown in Table 1, and the internal
control logic is shown in a simplified schematic
in Figure 6.
Bipolar
The gain and offset errors listed in the specifica-
tions may be adjusted to zero using the potenti-
ometers R and R (See Figure 5). If adjustment
isnotneed1ed,eith2erorbothpotsmaybereplaced
by a 50Ω, 1% metal film resistor.
Conversion Start
A conversion may be initiated by a logic transi-
tion on any of the three inputs: CE, CS R/C, as
shown in Table 1. The last of the three to reach
the correct state starts the conversion, so one,
two or all three may be dynamically controlled.
The nominal delay from each is the same and all
three may change state simultaneously. In order
to assure that a particular input controls the start
of conversion, the other two should be setup at
least 50ns earlier. Refer to the convert mode
timing specifications. The Convert Mode timing
diagram is shown in Figure 8.
To calibrate, connect the analog input signal to
pin 13 for a ±5V range or to pin 14 for a ±10V
1
range. First apply a DC input voltage ⁄2 LSB
above negative full scale which is –4.9988V for
the ±5V range or –9.9976V for the ±10V range.
Adjust the offset potentiometer R for flicker
between output codes 0000 0000 00100 and 0000
0000 0001. Next, apply a DC input voltage 11⁄2
LSBbelowpositivefullscalewhichis+4.9963V
forthe±5rangeor+9.9927Vforthe±10Vrange.
Adjust the gain potentiometer R for flicker
between codes 1111 1111 1110 an2d 1111 1111
1111.
The output signal STS is the status flag and goes
high only when a conversion is in progress.
While STS is high, the output buffers remain in
a high impedance state so that data can not be
read. Also, when STS is high, an additional Start
Convert will not reset the converter or reinitiate
a conversion. Note, if A0 changes state after a
conversion begins, an additional Start Convert
command will latch the new state of A0 and
possibly cause a wrong cycle length for that
conversion (8–versus 12–bits).
Alternative
The 100Ω potentiometer R2 provides gain adjust
for 10V and 20V ranges. In some applications, a
full scale of 10.24V (for and LSB of 2.5mV) or
20.48(foranLSBof5.0mV)ismoreconvenient.
For these, replace R2 by a 50Ω, 1% metal film
resistor.Thentoprovidegainadjustforthe10.24
range, add a 200Ω potentiometer and a 95Ω
fixed resistor, all in series with pin 13. For the
20.48V range, add a 500Ω potentiometer and a
200Ω fixed resistor in series with pin 14.
CE CS R/C 12/8 A0
OPERATION
0
x
x
1
0
0
x
x
0
0
0
0
x
x
x
x
x
x
x
x
1
0
0
x
x
0
1
0
1
0
1
x
0
1
None
None
Initiate 12–Bit Conversion
Initiate 8–Bit Conversion
Initiate 12–Bit Conversion
Initiate 8–Bit Conversion
Initiate 12–Bit Conversion
Initiate 8–Bit Conversion
Enable 12–Bit Output
CONTROLLING THE SPx74A
1
1
1
1
1
1
1
The SPx74A can be operated by most micropro-
cessor systems due to the control input pins and
on–chip logic. It may also be operated in the
“stand–alone” mode and enabled by the R/C
input pin. Full microprocessor control consists
of selecting an 8– or 12–bit conversion cycle,
initiating the conversion, and reading the output
data when ready. The output read has the options
of choosing either 12–bits at once or 8–bits
followed by 4–bits in a left–justified format. All
five control inputs are TTL/CMOS compatible
and include 12/8, CS, A0, R/C and CE. The use
0
0
0
0
0
1
1
1
Enable 8 MSB's Only
Enable 4 LSB's plus 4
Trailing Zeroes
Table 1. SPx74A Control Input Truth Table
11
SPECTRUM [ SPECTRUM MICROWAVE, INC. ]
