ADS5221 Datasheet, PDF (13/19 Pages) Texas Instruments – 12-Bit, 65MSPS Sampling, +3.3V ANALOG-TO-DIGITAL CONVERTER

The ADS5221 requires solid bypassing for all reference pins
to keep the effects of clock feedthrough to a minimum and to
achieve the specified levels of performance. Figure 7 to
Figure 10 show the recommended decoupling scheme. All
0.1µF capacitors must be located as close to the pins as
possible. In addition, pins REFT, V_REF, and REFB must be
decoupled with tantalum surface-mount capacitors (2.2µF,
4.7µF, 10µF or higher).
INTERNAL REFERENCE
There are two internal fixed reference modes and one
internal programmable reference mode as shown in Table I
and Figure 7 through Figure 9. Setting RSEL to ground (or
< 0.2V) provides an internal reference voltage of +1.0V at
V_REF pin, +2V at REFT, and +1V at REFB pin. In this case,
the input FSR is +2V peak-to-peak. Connecting RSEL to the
V_REF pin provides an internal reference voltage of +0.5V at
V_REF, +1.75V at REFT, and +1.25V at REFB. In this case, the
input FSR is +1V peak-to-peak. Setting the resister divider as
in Figure 9 provides an internal voltage between +0.5V and
+1V at V_REF, which is as follows:
V_REF = 0.5 • (1+R₂/R₁)
In this case, the voltage at REFT and REFB and input FSR
is calculated based on Table I.
RSEL
VREF
0.1µF
2.2µF
1V
Output
3.3V
ADS5221
2Ω
REFT
2Ω
REFB
10µF
2.2µF 15µF
2.2µF 15µF
402Ω
402Ω
FIGURE 7. Internal Reference Mode for V_REF = 1V.
RSEL
VREF
0.1µF 15µF
0.5V
Output
3.3V
ADS5221
2Ω
REFT
2Ω
REFB
10µF
2.2µF 15µF
2.2µF 15µF
402Ω
402Ω
RSEL
VREF
R₂
0.1µF 15µF
R₁
3.3V
ADS5221
2Ω
REFT
402Ω
10µF
2.2µF 15µF
2Ω
REFB
2.2µF 15µF 402Ω
FIGURE 9. Internal Reference Mode for V_REF = 0.5 • (1 + R₂/R₁).
EXTERNAL REFERENCE
For even more design flexibility, the ADS5221 can be oper-
ated with external references. Utilization of an external refer-
ence voltage may be considered for applications requiring
higher accuracy, improved temperature stability, or flexible
full-scale range. Particularly in multi-channel applications,
the use of a common external reference offers the benefit of
improving gain matching between converters. Setting RSEL
to AV_DD (+3.3V) provides an external reference mode for the
ADS5221. In this case, the internal V_REF amplifier is powered
down, and the V_REF pin requires an external reference
voltage between +0.5V to +1V to provide an input FSR
voltage of +1V to +2V. The REFT and REFB will appear with
the voltage as shown in Table I and input FSR is always
twice the voltage at the V_REF pin. A voltage reference
(REF1004 or TPS79225) and a single-supply amplifier
(OPA2234 or OPA4227) can be used to generate a precision
external reference.
AV_DD
RSEL
VREF
0.1µF 10µF
Input
0.5V to 1V
3.3V
ADS5221
2Ω
REFT
2Ω
REFB
402Ω
10µF
2.2µF 15µF
2.2µF 15µF 402Ω
FIGURE 8. Internal Reference Mode for V_REF = 0.5V.
FIGURE 10. External Reference Configuration.
ADS5221
13
SBAS262A
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