ADS5421 Datasheet, PDF (13/21 Pages) Burr-Brown (TI) – 14-Bit, 40MHz Sampling ANALOG-TO-DIGITAL CONVERTER

REFERENCE
REFERENCE OPERATION
Integrated into the ADS5421 is a bandgap reference circuit,
including logic that provides a +1V, +1.5V, or +2V reference
output by selecting the corresponding pin-strap configura-
tion. Table I lists a complete overview of the possible refer-
ence options and pin configurations.
Figure 8 shows the basic model of the internal reference
circuit. The functional blocks are a 1V bandgap voltage
reference, a selectable gain amplifier, the drivers for the top
and bottom reference (REFT, REFB), and the resistive refer-
ence ladder. The ladder resistance measures approximately
1kΩ between the REFT and REFB pins. The ladder is split
into two equal segments establishing a common-mode volt-
age at the ladder midpoint, labeled CM. The ADS5421
requires solid bypassing for all reference pins to keep the
effects of clock feedthrough to a minimum and to achieve the
specified level of performance. Figure 8 shows the recom-
mended decoupling scheme. All 0.1µF capacitors must be
located as close to the pins as possible. In addition, pins
REFT, CM, and REFB must be decoupled with tantalum
surface-mount capacitors (2.2µF or 4.7µF).
When operating the ADS5421 with the internal reference, the
effective full-scale input span for each of the inputs, IN and
IN, is determined by the voltage at the V_REF pin, given to:
(1)
Input Span (differential, each input) = V_REF = (REFT – REFB) in Vp-p
The top and bottom reference outputs can be used to provide
up to 1mA of current (sink or source) to external circuits.
Degradation of the differential linearity (DNL) and, conse-
quently, the dynamic performance, of the ADS5421 may
occur if this limit is exceeded.
USING EXTERNAL REFERENCES
For even more design flexibility, the ADS5421 can be oper-
ated with external references. The utilization of an external
reference voltage may be considered for applications requir-
ing higher accuracy, improved temperature stability, or a
continuous adjustment of the converter full-scale range.
Especially in multichannel applications, the use of a common
external reference offers the benefit of improving the gain
matching between converters. Selection between internal or
external reference operation is controlled through the V_REF
pin. The internal reference will become disabled if the voltage
applied to the V_REF pin exceeds +3.5V_DC. Once selected, the
ADS5421 requires two reference voltages: a top reference
voltage applied to the REFT pin and a bottom reference
voltage applied to the REFB pin (see Table I). As illustrated
in Figure 9, a micropower reference (REF1004) and a dual,
single-supply amplifier (OPA2234) can be used to generate
a precision external reference. Note that the function of the
range select pins, SEL1 and SEL2, are disabled while the
converter is operating in external reference mode.
DESIRED FULL-SCALE
RANGE (FSR)
(DIFFERENTIAL)
4Vp-p (+16dBm)
3Vp-p (+13dBm)
2Vp-p (+10dBm)
External Reference
CONNECT
SEL1 (PIN 45) TO:
GND
GND
V_REF
—
CONNECT
SEL2 (PIN 44) TO:
GND
+V_SA
GND
—
VOLTAGE AT V_REF
(PIN 46)
+2.0V
+1.5V
+1.0V
> +3.5V
VOLTAGE AT REFT
(PIN 52)
+3.5V
+3.25V
+3.0V
+2.75V to +4.5V
TABLE I. Reference Pin Configurations and Corresponding Voltages on the Reference Pins.
VOLTAGE AT REFB
(PIN 50)
+1.5V
+1.75V
+2.0V
+0.5V to +2.25V
SEL1 SEL2
45 44
REFBY
Range Select
and
61 _{Gain Amplifier}
0.1µF
Top
Reference
Driver
+1V_DC
Bandgap
Reference
ADS5421
V_REF
Bottom
Reference
Driver
46
0.1µF
REFT
52
500Ω
0.1µF
CM
51
500Ω
0.1µF
REFB
50
0.1µF
+
2.2µF
+
2.2µF
+
2.2µF
FIGURE 8. Internal Reference Circuit of the ADS5421 and Recommended Bypass Scheme.
ADS5421
13
SBAS237D
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