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SBAS341B − DECEMBER 2004 − REVISED APRIL 2005
of providing both a dc and a transient current. Figure 44
shows a simplified diagram of the internal circuitry of the
reference when the internal reference is disabled. As with
the input circuitry, switches S1 and S2 open and close as
shown by the timing in Figure 40.
REFERENCE INPUTS (VREFN, VREFP, VMID)
The ADS1602 can operate from an internal or external
voltage reference. In either case, the reference voltage
V
REF is set by the differential voltage between VREFN and
VREFP: VREF = (VREFP – VREFN). VREFP and VREFN
each use two pins, which should be shorted together.
VMID equals approximately 2.5V and is used by the
modulator. VCAP connects to an internal node and must
also be bypassed with an external capacitor.
ADS1602
S1
VREFP
VREFP
S2
INTERNAL REFERENCE (REFEN = LOW)
Ω
300
50pF
VREFN
VREFN
S1
To use the internal reference, set the REFEN pin low. This
activates the internal circuitry that generates the reference
voltages. The internal reference voltages are applied to
the pins. Good bypassing of the reference pins is critical
to achieve optimum performance and is done by placing
the bypass capacitors as close to the pins as possible.
Figure 43 shows the recommended bypass capacitor
values. Use high-quality ceramic capacitors for the smaller
values. Avoid loading the internal reference with external
circuitry. If the ADS1602 internal reference is to be used by
other circuitry, buffer the reference voltages to prevent
directly loading the reference pins.
Figure 44. Conceptual Internal Circuitry for the
Reference When REFEN = High
Figure 45 shows the recommended circuitry for driving
these reference inputs. Keep the resistances used in the
buffer circuits low to prevent excessive thermal noise from
degrading performance. Layout of these circuits is critical;
be sure to follow good high-speed layout practices. Place
the buffers, and especially the bypass capacitors, as close
to the pins as possible. VCAP is unaffected by the setting
on REFEN and must be bypassed when using the internal
or an external reference.
ADS1602
VREFP
µ
µ
10 F
0.1 F
VREFP
392Ω
µ
0.001 F
ADS1602
VMID
µ
0.1 F
VREFP
VREFP
OPA2822
µ
µ
0.1 F
10 F
µ
F
10
4V
µ
0.1
F
Ω
392
µ
0.1
F
VREFN
VREFN
0.001µF
µ
µ
0.1 F
10 F
VMID
OPA2822
10µF
2.5V
0.1µF
VCAP
392Ω
µ
0.1 F
0.001µF
AGND
VREFN
VREFN
OPA2822
Figure 43. Reference Bypassing When Using the
Internal Reference
1V
µ
µ
0.1 F
10
F
VCAP
EXTERNAL REFERENCE (REFEN = HIGH)
0.1µF
To use an external reference, set the REFEN pin high. This
deactivates the internal generators for VREFP, VREFN
and VMID, and saves approximately 25mA of current on
the analog supply (AVDD). The voltages applied to these
pins must be within the values specified in the Electrical
Characteristics table. Typically, VREFP = 4V, VMID = 2.5V
and VREFN = 1V. The external circuitry must be capable
AGND
Figure 45. Recommended Buffer Circuit When
Using an External Reference
17
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