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ADS807 Datasheet(PDF) 13 Page - Texas Instruments

Part No. ADS807
Description  12-Bit, 53MHz Sampling ANALOG-TO-DIGITAL CONVERTER
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Maker  TI1 [Texas Instruments]
Homepage  http://www.ti.com
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ADS807 Datasheet(HTML) 13 Page - Texas Instruments

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ADS807
13
SBAS072A
www.ti.com
REFERENCE OPERATION
The internal reference consists of a bandgap voltage refer-
ence, the drivers for the top and bottom reference, and the
resistive reference ladder. The bandgap reference circuit
includes logic functions that allow setting the analog input
swing of the ADS807 to a differential full-scale range of either
2Vp-p or 3Vp-p by simply tying the FSSEL pin to a LOW or
HIGH potential, respectively. While operating the ADS807 in
the external reference mode, the buffer amplifiers for the
REFT and REFB are disabled. The ADS807 has an internal
50k
Ω pull-down resistor at the range select pin (RSEL).
Therefore, this pin can be either hardwired to ground or left
unconnected, which will default the converter to a 2Vp-p full-
scale input range (FSR). While set for the 2Vp-p range, the
top and bottom reference voltages will be REFT = +3.0V and
REFB = +2.0V. Switching to the 3Vp-p range changes those
voltages to REFT = +3.25V and REFB = +1.75V. The
reference buffers can be utilized to supply up to 1mA (sink
and source) to external circuitry. To ensure proper operation
with any reference configuration, it is necessary to provide
solid bypassing at all reference pins in order to keep the
clock feedthrough to a minimum, as shown in Figure 6. Good
performance requires using 0.1
µF low inductance capaci-
tors. All bypassing capacitors should be located as close to
their respective pins as possible.
USING EXTERNAL REFERENCES
For even more design flexibility, the internal reference can
be disabled and an external reference voltage used. The
utilization of an external reference may be considered for
applications requiring higher accuracy, improved tempera-
ture performance, or a wide adjustment range of the
converter’s full-scale range. In multichannel applications, the
use of a common external reference has the benefit of
obtaining better matching and drift of the full-scale range
between converters. Figure 7 gives an example of an exter-
nal reference circuit using a single-supply, low-power, dual
op amp (OPA2234).
The external references can vary as long as the value of the
external top reference (REFT EXT) stays within the range of
VS – 1.70V and REFB + 0.4V, and the external bottom
reference (REFB EXT) stays within 1.70V and REFT – 0.4V.
Note that the function of the range selector pin (FSSEL) is
disabled while the converter operates in external reference
mode. Setting the ADS807 for external reference mode re-
quires the INT/EXT pin (pin 18) to be HIGH.
The logic level applied to the INT/EXT pin of the ADS807
determines if the converter operates with either the built-in
reference or external reference voltages. Because this function
pin has an internal 50k
Ω pull-up resistor, the default configu-
ration is external reference mode. Grounding this pin will
activate the internal reference option.
The input track-and-hold amplifier is differential. A positive
1Vp-p on the IN and its compliment, a negative
1Vp-p, on the IN (see Figure 3) results in 2Vp-p on the
output of the track-and-hold. Likewise, 2Vp-p on the IN and
0Vp-p on the IN (see Figure 4) results in 2Vp-p on the
output of the track-and-hold. Therefore, the reference volt-
ages, REFT and REFB, are the same for both differential
and single-ended inputs, see Table I.
The external references may be changed for different tasks.
The ADS807 will follow the external references with a latency
of 8 to 10 clock cycles. If it is desired to use INT/EXT and FSSEL
to change the configuration of a circuit for different tasks, a
large amount of time must be allowed. This time could be
hundreds of microseconds. Refer to the diagram on the front
page. Note that there is no disconnect for external references.
FIGURE 7. Example for an External Reference Driver Using the Dual, Single-Supply Op Amp, OPA2234.
REFT
CM
0.1
µF
(1)
(1)
(1)
REFB
ADS807
0.1
µF
0.1
µF
10
µF
NOTE: (1) Optional.
+
10
µF
+
10
µF
+
FIGURE 6. Recommended Bypassing for the Reference Pins.
OPA2234
A1
OPA2234
A2
R
3
R
4
R
2
R
1
+5V
+5V
0.1µF
10µF
> 1.70V
< 3.30V
Top Reference
Bottom Reference
+
4.7k
REF1004
+2.5V


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