AD1876

REV. A

–9–

corresponding pin. The capacitor is disconnected when SAMPLE

is taken LOW and the stored charge is used in the subsequent

A/D conversion. In order to limit the demands placed on the

external source by this high initial charging current, an internal

buffer amplifier is employed between the input and this capaci-

tance for a few hundred nanoseconds. During this time the

input pin exhibits typically 20 k

Ω input resistance, 10 pF input

capacitance and

±40 µA bias current. Next, the input is switched

directly to the now precharged capacitor and allowed to fully

settle, after which SAMPLE is taken LOW. During this time

the input sees only a 50 pF capacitor. Once the sample is taken,

the input is internally floated so that the external input source

sees a very high input resistance and a parasitic input capaci-

tance of typically only 2 pF. As a result, the only dominant input

characteristic which must be considered is the high current steps

which occur when the internal buffers are switched in and out.

In most cases, it is desirable to use external op amps to drive the

AD1876. For ac applications where low cost and low distortion

are desired, the AD711 may be used as shown in Figure 7. An-

other option is the 5532/5534 series. Care should always be

taken with op amp selection—many available op amps do not

meet the necessary low distortion requirements with even mod-

erate loading conditions.

1k

Ω

+12V

–12V

AD711

0.1µF

2

3

7

6

4

0.1µF

AGND

AGND SENSE

AD1876

10

8

9

1k

Ω

499

Ω

Figure 7.

TESTING THE AD1876

Analog Devices employs a high performance mixed signal VLSI

tester to verify the electrical performance of every AD1876. The

test system consists of two main sections, an input signal gen-

erator and a digital data and control section.

The stimulus section is responsible for providing a high purity,

noise-free, band limited tone to the input of the device. This in-

put frequency is 1.06 kHz. The test tone is passed through a

bandpass filter to remove distortion products and then buffered

by a high performance op amp. An external 5.000 V reference

voltage is also supplied by this section.

The control section of the test equipment provides an external

clock and the control signals for calibration, conversion and data

transmission. This section of the tester also contains the pro-

cessing unit that calculates the actual performance of the device

under test.

The test procedure consists of the following steps. First, the

device is calibrated by its on-board controller. Next, the device

under test digitizes the input waveform. This conversion is

performed at a 96 kSPS rate and transmits the resulting serial

data to the tester. The tester performs an FFT on the test data

and determines the actual performance of the device.

AC PERFORMANCE

Using the aforementioned test methodology, ac performance

of the AD1876 is measured. AC parameters, which include

S/(N+D), THD, etc., reflect the AD1876’s effect on the spec-

tral content of the analog input signal. Figures 11 through 15

provide information on the AD1876’s ac performance under a

variety of conditions.

As a general rule, averaging the results from several conversions

reduces the effects of noise and, therefore, improves such pa-

rameters as S/(N+D) and THD. AD1876 performance is opti-

mized by operating the device at its maximum sample rate of

100 kSPS and digitally filtering the resulting bit stream to the

desired signal bandwidth. This succeeds in distributing noise

over a wider frequency range, thus reducing the noise density in

the frequency band of interest. This subject is discussed in the

following section.

OVERSAMPLING AND NOISE FILTERING

The Nyquist rate for a converter is defined as one-half its sam-

pling rate. This is established by the Nyquist theorem, which

requires that a signal be sampled at a rate corresponding to at

least twice its widest bandwidth of interest in order to preserve

the information content. Oversampling is a conversion tech-

nique in which the sampling frequency is an integral (2 or more)

multiple of twice the frequency bandwidth of interest. In audio

applications, the AD1876 can operate at a 2

× oversampling rate.

In quantized systems, the information content of the analog in-

put is represented in the frequency spectrum from dc to the

Nyquist rate of the converter. Within this same spectrum are

higher frequency aliased noise components. Antialias, or low-

pass, filters are used at the input to the ADC to remove the por-

tion of these noise components attributed to high frequency

analog input noise. However, wideband noise contributed by the

AD1876 will not be reduced by the antialias filter. The AD1876

contributed noise is evenly distributed from dc to the Nyquist

rate, and this fact can be used to minimize its overall effect.

The AD1876 contributed noise effects can be reduced by

oversampling—sampling at a rate higher than defined by the

Nyquist theorem. This spreads the noise energy over a distribu-

tion of frequencies wider than the frequency band of interest,

and by judicious selection of a digital filter, noise frequencies

outside the bandwidth of interest may be eliminated. The pro-

cess of quantization inherently produces noise, known as quanti-

zation noise. The magnitude of this noise is a function of the

resolution of the converter, and manifests itself as a limit to the

theoretical signal-to-noise ratio achievable. This limit is de-

audio bandwidth applications, the AD1876 is capable of operat-

ing at a 2

× oversample rate (96 kSPS), which typically produces

an improvement in S/(N+D) of 3 dB compared with operating

at the Nyquist conversion rate of 48 kSPS. Oversampling has

another advantage as well; the demands on the antialias filter are