ADS58C48IPFPR datasheet(62/70 Pages) TI

62 / 70 page

SNR = 10Log

ADS58C48

SLAS689 – MAY 2010

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BOARD DESIGN CONSIDERATIONS

For EVM board information, refer to the ADS58C48 EVM User's Guide (SLAU313).

Grounding

A single ground plane is sufficient to give good performance, provided the analog, digital, and clock sections of

the board are cleanly partitioned. See the EVM User's Guide (SLAU313) for details on layout and grounding.

Exposed Pad

In addition to providing a path for heat dissipation, the pad is also electrically connected to analog and digital

ground internally. So, it is necessary to solder the exposed pad to the ground plane for best thermal and

electrical performance. For detailed information, see application notes.

DEFINITION OF SPECIFICATIONS

Analog Bandwidth – The analog input frequency at which the power of the fundamental is reduced by 3 dB with

respect to the low-frequency value.

Aperture Delay – The delay in time between the rising edge of the input sampling clock and the actual time at

which the sampling occurs. This delay is different across channels. The maximum variation is specified as

aperture delay variation (channel-to-channel).

Aperture Uncertainty (Jitter) – The sample-to-sample variation in aperture delay.

Clock Pulse Width/Duty Cycle – The duty cycle of a clock signal is the ratio of the time the clock signal remains

at a logic high (clock pulse width) to the period of the clock signal. Duty cycle is typically expressed as a

percentage. A perfect differential sine-wave clock results in a 50% duty cycle.

Maximum Conversion Rate – The maximum sampling rate at which specified operation is given. All parametric

testing is performed at this sampling rate unless otherwise noted.

Minimum Conversion Rate – The minimum sampling rate at which the ADC functions.

Differential Nonlinearity (DNL) – An ideal ADC exhibits code transitions at analog input values spaced exactly

1 LSB apart. The DNL is the deviation of any single step from this ideal value, measured in units of LSBs.

Integral Nonlinearity (INL) – The INL is the deviation of the ADC transfer function from a best fit line determined

by a least squares curve fit of that transfer function, measured in units of LSBs.

Gain Error – Gain error is the deviation of the ADC actual input full-scale range from its ideal value. The gain

error is given as a percentage of the ideal input full-scale range. Gain error has two components: error as a

result of reference inaccuracy and error as a result of the channel. Both errors are specified independently as

EGREF and EGCHAN.

To a first-order approximation, the total gain error is ETOTAL ~ EGREF + EGCHAN.

For example, if ETOTAL = ±0.5%, the full-scale input varies from (1 – 0.5/100) x FSideal to (1 + 0.5/100) x FSideal.

Offset Error – The offset error is the difference, given in number of LSBs, between the ADC actual average idle

channel output code and the ideal average idle channel output code. This quantity is often mapped into millivolts.

Temperature Drift – The temperature drift coefficient (with respect to gain error and offset error) specifies the

change per degree Celsius of the parameter from TMIN to TMAX. It is calculated by dividing the maximum deviation

of the parameter across the TMIN to TMAX range by the difference TMAX – TMIN.

Signal-to-Noise Ratio – SNR is the ratio of the power of the fundamental (PS) to the noise floor power (PN),

excluding the power at dc and the first nine harmonics.

(1)

SNR is either given in units of dBc (dB to carrier) when the absolute power of the fundamental is used as the

reference, or dBFS (dB to full-scale) when the power of the fundamental is extrapolated to the converter

full-scale range.

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