AD7883

REV. 0

–7–

+

–

R1

10k

Ω

AGND

AD7883*

R2

500

Ω

R3

10k

Ω

R5

10k

Ω

R4

10k

Ω

*ADDITIONAL PINS OMITTED FOR CLARITY

Figure 9. Offset and Full-Scale Adjust Circuit

Unipolar Adjustments

In the case of the 0 V to 3.3 V unipolar input configuration, uni-

polar offset error must be adjusted before full-scale error. Ad-

justment is achieved by trimming the offset of the op amp

driving the analog input of the AD7883. This is done by apply-

adjusting the op amp offset voltage until the ADC output code

flickers between 0000 0000 0000 and 0000 0000 0001. For full-

scale adjustment, an input voltage of 3.2988 V (FS–3/2 LSBs) is

between 1111 1111 1110 and 1111 1111 1111.

Bipolar Adjustments

Bipolar zero and full-scale errors for the bipolar input configura-

tion of Figure 5 are adjusted in a similar fashion to the unipolar

case. Again, bipolar zero error must be adjusted before full-scale

error. Bipolar zero error adjustment is achieved by trimming the

offset of the op amp driving the analog input of the AD7883

while the input voltage is 1/2 LSB below ground. This is done

ure 9 and adjusting the op amp offset voltage until the ADC

output code flickers between 0111 1111 1111 and 1000 0000

0000. For full-scale adjustment, an input voltage of 3.2988 V

output code flickers between 1111 1111 1110 and 1111 1111

1111.

DYNAMIC SPECIFICATIONS

The AD7883 is specified and tested for dynamic performance

specifications as well as traditional dc specifications such as inte-

gral and differential nonlinearity. The ac specifications are re-

quired for signal processing applications such as speech

recognition, spectrum analysis and high speed modems. These

applications require information on the ADC’s effect on the

spectral content of the input signal. Hence, the parameters for

which the AD7883 is specified include SNR, harmonic distor-

tion, intermodulation distortion and peak harmonics. These

terms are discussed in more detail in the following sections.

Signal-to-Noise Ratio (SNR)

SNR is the measured signal-to-noise ratio at the output of the

ADC. The signal is the rms magnitude of the fundamental.

Noise is the rms sum of all the nonfundamental signals up to

half the sampling frequency (FS/2) excluding dc. SNR is depen-

dent upon the number of quantization levels used in the digiti-

zation process; the more levels, the smaller the quantization

noise. The theoretical signal to noise ratio for a sine wave input

is given by:

SNR = (6.02 N + 1.76) dB

(1)

where N is the number of bits.

Thus for an ideal 12-bit converter, SNR = 74 dB.

The output spectrum from the ADC is evaluated by applying a

sampled at a 50 kHz sampling rate. A Fast Fourier Transform

(FFT) plot is generated from which the SNR data can be ob-

tained. Figure 10 shows a typical 2048 point FFT plot of the

AD7883 with an input signal of 2.5 kHz and a sampling fre-

quency of 50 kHz. The SNR obtained from this graph is 71 dB.

It should be noted that the harmonics are taken into account

when calculating the SNR.

0

–30

–60

–90

–120

0

2.5

25

FREQUENCY – kHz

INPUT FREQUENCY = 2.5kHz

SAMPLE FREQUENCY = 50kHz

SNR = 71.4dB

T

Figure 10. FFT Plot

Effective Number of Bits

The formula given in Equation 1 relates the SNR to the number

of bits. Rewriting the formula, as in Equation 2, it is possible to

get a measure of performance expressed in effective number of

bits (N).

N

=

SNR –1.76

6.02

(2)

The effective number of bits for a device can be calculated di-

rectly from its measured SNR.

Figure 11 shows a plot of effective number of bits versus input

frequency for an AD7883 with a sampling frequency of 50 kHz.

The effective number of bits typically remains better than 11.5

for frequencies up to 12 kHz.