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SBAS274E − MARCH 2003 − REVISED JUNE 2004
Characteristics table. Typically VREFP = 4V, VMID = 2.5V
and VREFN = 1V. The external circuitry must be capable
of providing both a dc and a transient current. Figure 13
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 in Figure 9.
REFERENCE INPUTS (VREFN, VREFP, VMID)
The ADS1605/6 can operate from an internal or external
voltage reference. In either case, the reference voltage VREF
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. For the best analog
performance, it is recommended that an external reference
voltage (VREF) of 3.0V be used.
ADS1605
ADS1606
S1
VREFP
VREFP
S2
INTERNAL REFERENCE (REFEN = LOW)
Ω
300
50pF
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 12 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 ADS1605/6 internal reference is to be used
by other circuitry, buffer the reference voltages to prevent
directly loading the reference pins.
VREFN
VREFN
S1
Figure 13. Conceptual Internal Circuitry for the
Reference When REFEN = High
Figure 14 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,
make 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.
ADS1605
ADS1606
VREFP
µ
µ
0.1 F
VREFP
10 F
Ω
392
µ
0.001
F
µ
22 F
ADS1605
ADS1606
VMID
µ
22 F
µ
0.1 F
VREFP
VREFP
OPA2822
µ
µ
0.1 F
10 F
µ
10
F
4V
0.1µF
Ω
392
µ
22 F
µ
0.1
F
VREFN
VREFN
µ
0.001
F
µ
22 F
µ
22
F
µ
µ
0.1 F
10 F
VMID
OPA2822
µ
10
F
2.5V
VCAP
µ
0.1
F
µ
0.1 F
Ω
392
µ
0.001
F
AGND
22µF
VREFN
VREFN
OPA2822
Figure 12. Reference Bypassing When Using the
Internal Reference
1V
µ
µ
µ
10
F
0.1
0.1
F
F
EXTERNAL REFERENCE (REFEN = HIGH)
VCAP
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
AGND
Figure 14. Recommended Buffer Circuit When
Using an External Reference
19
BB [ BURR-BROWN CORPORATION ]
