AD7652 datasheet(15/23 Pages) AD

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REV. PrA

PRELIMINARY TECHNICAL DATA

AD7667

–15–

The AD829 is another alternative where high-frequency

(above 100 kHz) performance is not required. In gain of

1, it requires an 82 pF compensation capacitor.

The AD8610 is another option where low bias current is

needed in low-frequency applications.

Voltage Reference Input

The AD7667 allows the choice of either

an internal 2.5 V

voltage reference or an external 2.5 V reference.

To use the internal reference along with the internal buffer,

PDREF and PDBUF should both be LOW. This will pro-

duce a voltage on REFBUFIN of 1.25 V and the buffer’s

gain will be 2, resulting in a 2.5 V reference on REF pin.

To use an external reference along with the internal

buffer, PDREF should be HIGH and PDBUF should be

LOW. This powers down the internal reference and allows

for the 2.5 V reference to be applied to REFBUFIN. In

this mode the buffer’s gain is 1.

To use both external reference, PDREF and PDBUF

should both be HIGH. The reference input should be

applied to REF.

It is useful to decouple the REFBUFIN pin with a 100 nF

ceramic capacitor. The output impedance of the REFBUFIN

pin is 4 k . Thus, the 100 nF capacitor provides an RC filter

for noise reduction.

It should be noted that the internal reference and internal

buffer are independent of the power down (PD) pin of the

part. Powering down the part does not power down the inter-

nal reference or the internal buffer. Furthermore, powering

down the internal reference and internal buffer, as well as

powering them up, requires time. This is due to the fact that

we have charging and discharging capacitors on the REF

which require some settling time. Therefore, for applications

requiring low power, there will always be a typical of 10 mW

of power dissipated when using the internal reference and

internal buffer even during times with no conversions.

The internal reference is temperature compensated to

2.5V ± TBD mV. The reference is trimmed to provide

a typical drift of TBD ppm/ C. This typical drift char-

acteristic is shown in Figure TBD. For improved drift

performance, an external reference such as the AD780

can be used.

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TO BE

SUPPLIED

Figure TBD

For the external reference, the voltage reference input REF

of the AD7667 has a dynamic input impedance; it should

therefore be driven by a low-impedance source with an

efficient decoupling between REF and REFGND inputs.

This decoupling depends on the choice of the voltage refer-

ence but usually consists of a 1 µF ceramic capacitor and a

low ESR tantalum capacitor connected to the REF and

REFGND inputs with minimum parasitic inductance. 47

µF is an appropriate value for the tantalum capacitor when

using either the internal reference of one of the recom-

mended reference voltages:

− The low noise, low temperature drift ADR421 and

AD780 voltage references

− The low power ADR291 voltage reference

− The low cost AD1582 voltage reference

For applications using multiple AD7667s, it is more effective

to buffer the reference voltage using the internal buffer. To

do so, PDREF should be HIGH, and PDBUF should be low.

Care should also be taken with the reference temperature

coefficient of the voltage reference which directly affects the

full-scale accuracy if this parameter matters. For instance, a

±15 ppm/°C tempco of the reference changes the full scale by

±1 LSB/°C.

VREF , as mentioned in the specification table, could be increased

to AVDD – 1.85 V. The benefit here is the increased SNR

obtained as a result of this increase. Since the input range is

defined in terms of VREF, this would essentially increase the

range to make it a 0 to 3 V input range with an AVDD above

4.85 V. One of the benefits here is the additional SNR ob-

tained as a result of this increase. The theoretical

improvement as a result of this increase in reference is 1.58

dB (20 log [3/2.5]). Due to the theoretical quantization

noise, however, the observed improvement is approximately

1 dB. The AD780 can be selected with a 3 V reference

voltage.

The TEMP pin, which measures the temperature of the

AD7667, can be used as follows. Refer to figure TBD to

see the connectivity. The output of the TEMP pin is ap-

plied to one of the inputs of the analog switch (ADG779).

The other input, as shown is the analog signal. The output

of the switch is connected to the AD8021 which is config-

ured as a follower. The output of the op-amp is applied to

the IN pin. Refer to the Specification Table for the appro-

priate values related to the TEMP pin. This configuration

could be very useful to improve the calibration accuracy

over the temperature range.

AD8021

AD7667

ANALOG INPUT

(UNIPOLAR)

temperature

sensor

ADG779

TEMP

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