ADS1606IPAPR datasheet(19/38 Pages) TI

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ADS1605

ADS1606

SBAS274H − MARCH 2003 − REVISED MAY 2007

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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 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 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 (VREF) 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.

µF

µF0.1µF

µF

0.1

µF

0.1

µF

0.1

µF

0.1

µF

ADS1605

ADS1606

AGND

VREFP

VMID

VCAP

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.

ADS1605

ADS1606

VREFP

50pF

300

Ω

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.

µF

0.1

µF

0.1

µF

0.1

µF

0.1

µF

0.1

µF

VREFP

VMID

VCAP

VREFN

392

Ω

OPA 2822

A DS1605

A DS1606

0.001

µF

392

Ω

OPA 2822

0.001

µF

392

Ω

OPA 2822

0.001

µF

2.5V

AGND

Figure 14. Recommended Buffer Circuit When

Using an External Reference

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