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 voltages, REFT and
REFB, are the same for both differential and single-ended
inputs, as shown in Table I.
USING EXTERNAL REFERENCES
For even more design flexibility, the internal reference can
be disabled and an external reference voltage used. Driving
both channels with an external reference offers the best
performance, as it allows the channels to maintain balance.
The utilization of an external reference may be considered
for applications requiring higher accuracy, improved tem-
perature 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
external reference circuit using a single-supply, low-power,
dual op amp (OPA2234).
INPUT
REFERENCE IN (Pin-50, 63) IN (Pin-51, 62) REFT REFB
2Vp-p Differential
Internal
2V to 3V
3V to 2V
+3V
+2V
1Vp-p Times 2 Inputs
or External
2Vp-p Single-Ended
2Vp-p Times 1 Input
Internal
or External
1.5V to 3.5V
2.5VDC
+3V
+2V
3Vp-p Differential
Internal
1.75V to 3.35V 3.25V to 1.75V +3.25V +1.75V
The external references can vary as long as the value of the
external top reference (REFT) stays within the range of
VS – 1.70V and REFB + 0.4V, and the external bottom
reference (REFB) stays within 1.70V and REFT – 0.4V.
Note that the function of the range selector pin (SEL) is
disabled while the converter operates in external reference
mode. Setting the ADS2806 for external reference mode
requires the INT/EXT pin (pin 18) to be HIGH.
1.5Vp-p Times 2 Inputs
or External
3Vp-p Single-Ended
3Vp-p Times 1 Input
Internal
or External
1V to 4V
2.5VDC
+3.25V +1.75V
TABLE I. Reference Voltages for Input Signal Ranges.
The external references may be changed for different tasks.
The ADS2806 will follow the external references with a
latency of 8 to 10 clock cycles. If it is desired to use INT/EXT
and SEL 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. If it is desired to switch between inter-
nal and external references, disconnect switches must be
added between the external references and the ADS2806.
The logic level applied to the INT/EXT pin of the ADS2806
determines if the converter operates with either the built-in
reference or external reference voltages. Due to this func-
tion pin having an internal 50kΩ pull-up resistor, the default
configuration is external reference mode. Grounding this pin
will activate the internal reference option.
+5V
+5V
< 3.30V
OPA2234
Top Reference
A1
4.7kΩ
R3
R4
R1
+
REF1004
+2.5V
10µF
> 1.70V
OPA2234
Bottom Reference
0.1µF
R2
A2
One Channel of Two
FIGURE 7. Example for an External Reference Driver Using the Dual, Single-Supply Op Amp, OPA2234.
ADS2806
SBAS178B
14
www.ti.com
BB [ BURR-BROWN CORPORATION ]
