ADS7811_15 Datasheet, PDF (8/15 Pages) Texas Instruments – 16-Bit 250kHz Sampling CMOS ANALOG-to-DIGITAL CONVERTER

a conversion, R/C should be taken LOW at least 100ns
before CS is taken LOW. R/C and/or CS should be taken
HIGH during the early part of the conversion, preferably
within 200ns of the start of the conversion. If these times are
not observed, then there is risk that the transition of these
digital signals may affect the conversion result.
The three NAND gates shown in Figure 1 can be used to
generate R/C and CS signals from a single negative going
pulse. The pulse must not be longer than 3.3µs or a second
conversion may be initiated immediately after the first.
BUSY
BUSY goes LOW when a conversion is started and remains
LOW throughout the conversion. Just prior to BUSY going
HIGH, the digital outputs become active with the conversion
result. Time t₁₁, shown in Figure 2, should provide adequate
time for the ADS7811 to drive the digital outputs to a valid
logic state before BUSY rises. As shown in Figure 1 and 2,
the rising edge of BUSY can be used to latch the digital
result into an external component.
DIGITAL OUTPUT
The ADS7811’s digital output is in Binary Two’s Comple-
ment (BTC) format. Table III shows the relationship be-
tween the digital output word and analog input voltage under
ideal conditions.
DESCRIPTION
Full Scale Range
Least Significant
Bit (LSB)
+Full Scale
(2.5V – 1LSB)
Midscale
One LSB below
Midscale
–Full Scale
ANALOG
INPUT
±2.5V
76µV
DIGITAL OUTPUT
BINARY TWO’S COMPLEMENT
BINARY CODE
HEX CODE
2.499924V 0111 1111 1111 1111
7FFF
0V
0000 0000 0000 0000
0000
–76 µV
–2.5V
1111 1111 1111 1111
1000 0000 0000 0000
FFFF
8000
Table III. Ideal Input Voltages and Output Codes.
REFERENCE
The ADS7811 can be operated with the internal 2.5V refer-
ence or an external reference. By applying an external
reference to the REF pin, the internal reference is bypassed.
The reference voltage at REF is buffered internally.
The voltage at the reference input sets the full-scale range of
the converter. With the internal 2.5V reference, the input
range is ±2.5V. Thus, the input range of the converter’s
analog input is simply ±V_REF, where V_REF is the voltage at
the reference input. Because of internal gain and offset error,
the input range will not be exactly ±V_REF. The full-scale
error of the converter with an external reference will typi-
cally be 0.25% or less. The bipolar zero error will be similar
to that listed in the Electrical Characteristics Table. The
range for the external reference is 2.3V to 2.7V. While the
ADS7811 will operate using an external reference, the
specifications are only ensured when the internal reference
is used.
REF PIN
The REF pin itself should be bypassed with a 0.1µF ceramic
capacitor in parallel with a 2.2µF tantalum capacitor. While
both capacitors should be physically close to the ADS7811,
it is very important that the ceramic capacitor be placed as
close as possible.
The REF voltage should not be used to drive a large load or
any load which is dynamic. A large load will reduce the
reference voltage and the corresponding input range of the
converter. A dynamic load will modulate the reference
voltage and this modulation will be present in the converter’s
output data.
CAP PIN
The voltage on the CAP pin is the output of the reference
buffer. This pin should be bypassed with a 0.1µF ceramic
capacitor in parallel with a 2.2µF tantalum capacitor. While
both capacitors should be physically close to the ADS7811,
it is very important that the ceramic capacitor be placed as
close as possible.
The CAP pin connects to the internal reference buffer and
directly to the binary weighted capacitor array of the con-
verter. Thus, the signal at the CAP pin has high-frequency
glitches which occur at each bit decision. For this reason, the
CAP voltage should not be used to provide a reference
voltage for external circuitry.
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ADS7811
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