Datasheet

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SBAS274H − MARCH 2003 − REVISED MAY 2007
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19
REFERENCE INPUTS (VREFN, VREFP, VMID)
The ADS1605/6 can operate from an internal or exter-
nal voltage reference. In either case, the reference volt-
age V
REF
is set by the differential voltage between
VREFN and VREFP: V
REF
= (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 in-
ternal node and must also be bypassed with an external
capacitor. For the best analog performance, it is recom-
mended that an external reference voltage (V
REF
) of
3.0V be used.
INTERNAL REFERENCE (REFEN = LOW)
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 ca-
pacitors for the smaller values. Avoid loading the inter-
nal 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.
10
µ
F
22
µ
F0.1
µ
F
22
µ
F
22
µ
F
0.1
µ
F
10
µ
F 0.1
µ
F
10
µ
F 0.1
µ
F
0.1
µ
F
ADS1605
ADS1606
AGND
VREFP
VREFP
VMID
VCAP
VREFN
VREFN
Figure 12. Reference Bypassing When Using the
Internal Reference
EXTERNAL REFERENCE (REFEN = HIGH)
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 ap-
plied 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 cir-
cuitry must be capable of providing both a dc and a tran-
sient 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.
S
1
ADS1605
ADS1606
S
2
VREFP
50pF300
Ω
S
1
VREFN
VREFP
VREFN
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 cir-
cuits 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 ref-
erence.
10
µ
F
22
µ
F
0.1
µ
F
22
µ
F
22µF
0.1µF
10
µ
F 0.1
µ
F
10
µ
F
0.1
µ
F
0.1
µ
F
VREFP
VREFP
VMID
VCAP
VREFN
VREFN
392
Ω
OPA2822
ADS1605
ADS1606
0.001
µ
F
4V
392
Ω
OPA2822
0.001
µ
F
1V
392
Ω
OPA2822
0.001
µ
F
2.5V
AGND
Figure 14. Recommended Buffer Circuit When
Using an External Reference