AD7667AST datasheet(20/29 Pages) AD

20 / 29 page

AD7667

Rev. 0 | Page 19 of 28

•

The noise generated by the driver amplifier needs to be

kept as low as possible in order to preserve the SNR and

transition noise performance of the AD7667. The noise

coming from the driver is filtered by the AD7667 analog

input circuit 1-pole low-pass filter made by R1 and C2 or

by the external filter, if one is used. The SNR degradation

due to the amplifier is

⎟

⎠

⎞

⎜

⎝

⎛

−

)

(

784

log

LOSS

SNR

where:

f–3dB

is the input bandwidth of the AD7667 (13 MHz) or

the cutoff frequency of the input filter (3.9 MHz), if

one is used.

is the noise factor of the amplifier (+1 in buffer

configuration).

is the equivalent input noise voltage of the op amp, in

nV/√Hz.

For instance, a driver with an equivalent input noise of

2 nV/√Hz, like the AD8021 with a noise gain of +1 when

configured as a buffer, degrades the SNR by only 0.13 dB

when using the filter in Figure 26 and by 0.43 dB without.

•

The driver needs to have a THD performance suitable to

that of the AD7667. Figure 15 gives the THD versus

frequency that the driver should exceed.

The AD8021 meets these requirements and is appropriate for

almost all applications. The AD8021 needs a 10 pF external

compensation capacitor that should have good linearity as an

NPO ceramic or mica type. Moreover, the use of a noninverting

+1 gain arrangement is recommended and helps to obtain the

best signal-to-noise ratio.

The AD8022 could also be used if a dual version is needed and

gain of 1 is present. The AD829 is an alternative in applications

where high frequency (above 100 kHz) performance is not

required. In gain of 1 applications, it requires an 82 pF

compensation capacitor. The AD8610 is an option when low

bias current is needed in low frequency applications.

Voltage Reference Input

The AD7667 allows the choice of either a very low temperature

drift internal voltage reference or an external 2.5 V reference.

Unlike many ADCs with internal references, the internal

reference of the AD7667 provides excellent performance and

can be used in almost all applications.

To use the internal reference along with the internal buffer,

PDREF and PDBUF should both be LOW. This produces 1.2 V

on REFBUFIN which, amplified by the buffer, results in a 2.5 V

reference on the REF pin.

The output impedance of REFBUFIN is 11 kΩ (minimum)

when the internal reference is enabled. It is necessary to

decouple REFBUFIN with a ceramic capacitor greater than

10 nF. Thus the capacitor provides an RC filter for noise

reduction.

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 the 2.5 V

reference to be applied to REFBUFIN.

To use an external reference directly on REF pin, PDREF and

PDBUF should both be HIGH.

PDREF and PDBUF power down the internal reference and the

internal reference buffer, respectively. Note that the PDREF and

PDBUF input current should never exceed 20 mA. This could

eventually occur when input voltage is above AVDD (for

instance, at power-up). In this case, a 100 Ω series resistor is

recommended.

The internal reference is temperature compensated to 2.5 V

±7 mV. The reference is trimmed to provide a typical drift of

3 ppm/°C . This typical drift characteristic is shown in

Figure 22. For improved drift performance, an external

reference, such as the AD780, can be used.

The AD7667 voltage reference input REF has a dynamic input

impedance; it should therefore be driven by a low impedance

source with efficient decoupling between the REF and

REFGND inputs. This decoupling depends on the choice of the

voltage reference but usually consists of a low ESR tantalum

capacitor connected to REF and REFGND with minimum

parasitic inductance. A 10 µF (X5R, 1206 size) ceramic chip

capacitor (or 47 µF tantalum capacitor) is appropriate when

using either the internal reference or one of these

recommended reference voltages:

•

The low noise, low temperature drift ADR421 and AD780

•

The low power ADR291

•

The low cost AD1582

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