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AD7641ACP

18-Bit, 2 MSPS SAR ADC

Analog Devices 

AD7641

• The noise generated by the driver amplifier needs to be

kept as low as possible to preserve the SNR and transition

noise performance of the AD7641. The noise coming from

the driver is filtered by the AD7641 analog input circuit

1-pole, low-pass filter made by RIN and CIN or by the

external filter, if one is used. The SNR degradation due

to the amplifier is

⎜⎛

SNRLOSS = 20log⎜⎜

⎜⎜⎝

⎟⎞

30

⎟

900 +

πf −3dB

2

(NeN+ )2

+

πf −3dB

2

(NeN− )2

⎟

⎟⎟⎠

where:

f–3dB is the input bandwidth of the AD7641 (50 MHz) or

the cutoff frequency of the input RC filter shown in Figure 23

(3.9 MHz), if one is used.

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

configuration).

eN+ and eN− are the equivalent input voltage noise densities

of the op amps connected to IN+ and IN−, in nV/√Hz.

This approximation can be used when the resistances used

around the amplifier are small. If larger resistances are

used, their noise contributions should also be root-sum

squared.

For instance, when using op amps with an equivalent input

noise density of 2.1 nV/√Hz, such as the AD8021, with a

noise gain of 1 when configured as a buffer, degrades the

SNR by only 0.25 dB when using the RC filter in Figure 23,

and by 2.5 dB without it.

• The driver needs to have a THD performance suitable to

that of the AD7641. Figure 13 gives the THD vs. 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 can also be used when a dual version is needed

and a gain of 1 is present. The AD829 is an alternative in

applications where high frequency (above 100 kHz) performance is

not required. In applications with a gain of 1, an 82 pF

compensation capacitor is required. The AD8610 is an option

when low bias current is needed in low frequency applications.

Single-to-Differential Driver

For applications using unipolar analog signals, a single-ended-

to-differential driver, as shown in Figure 26, allows for a

differential input into the part. This configuration, when

provided an input signal of 0 to VREF, produces a differential

±VREF with midscale at VREF/2. The 1-pole filter using R = 10 Ω

and C = 1 nF provides a corner frequency of 16 MHz.

If the application can tolerate more noise, the AD8139

differential driver can be used.

ANALOG INPUT

(UNIPOLAR 0V TO 2.048V)

U1

AD8021

10pF

590Ω

590Ω

15Ω

2.7nF

5kΩ

5kΩ

100nF

U2

AD8021

10pF

15Ω

2.7nF

IN+

AD7641

IN– REF

10µF

Figure 26. Single-Ended-to-Differential Driver Circuit

(Internal Reference Buffer Used)

VOLTAGE REFERENCE INPUT

The AD7641 allows the choice of either a very low temperature

drift internal voltage reference or an external reference.

Unlike many ADCs with internal references, the internal

reference of the AD7641 provides excellent performance and

can be used in almost all applications.

Internal Reference (PDBUF = Low, PDREF = Low)

To use the internal reference, the PDREF and PDBUF inputs

must both be low. This produces a 1.2 V band gap output on

REFBUFIN, which is amplified by the internal buffer and

results in a 2.048 V reference on the REF pin.

The internal reference is temperature compensated to 2.048 V ±

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

10 ppm/°C. This typical drift characteristic is shown in Figure 7.

The output resistance of REFBUFIN is 6.33 kΩ (minimum)

when the internal reference is enabled. It is necessary to

decouple this with a ceramic capacitor greater than 100 nF.

Therefore, the capacitor provides an RC filter for noise reduction.

Because the output impedance of REFBUFIN is typically

6.33 kΩ, relative humidity (among other industrial contaminates)

can directly affect the drift characteristics of the reference.

Typically, a guard ring is used to reduce the effects of drift

under such circumstances.

Rev. 0 | Page 18 of 28

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