ADS1625IPAPRG4 datasheet(19/37 Pages) TI

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ADS1625

ADS1626

SBAS280E − JUNE 2003 − REVISED MAY 2007

www.ti.com

REFERENCE INPUTS (VREFN, VREFP, VMID)

The ADS1625 can operate from an internal or external

voltage reference. In either case, 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.

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 capacitors for the smaller

values. Avoid loading the internal reference with external

circuitry. If the ADS1625/6 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

ADS1625

ADS1626

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 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

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.

ADS1625

ADS1626

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 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.

0.1

VREFP

VMID

VCAP

VREFN

392

Ω

OPA2822

AD S1625

AD S1626

0.001

392

Ω

OPA2822

0.001

392

Ω

OPA2822

0.001

2.5V

AGND

µF

µ F

µF

µ F

Figure 14. Recommended Buffer Circuit When

Using an External Reference

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