Input Frequency (kHz)

0

20

40

60

80

100

9

9.5

10

10.5

11

11.5

12

D025

TLV2553

SLAS354C – SEPTEMBER 2001 – REVISED SEPTEMBER 2015

www.ti.com

9 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component

specification, and TI does not warrant its accuracy or completeness. TI’s customers are

responsible for determining suitability of components for their purposes. Customers should

validate and test their design implementation to confirm system functionality.

9.1 Application Information

As with most SAR ADCs, the inputs of the TLV2553 are not high-impedance ports. At the start of the sampling

phase, the selected input channel experiences a load current, as the internal analog switches close and the

sampling capacitor starts to charge (or discharge). This load current decays over time and varies in a nonlinear

fashion with respect to input voltage.

The load current is supplied by the input signal source which has non-zero output impedance. As a result, the

load current drops non-zero voltage across the output impedance of the signal source creating a time-decaying,

nonlinear error between the signal source output and the ADC input. This is called sampling error, and if the

sampling error does not decay to less than 1 LSB before the end of the sampling window when the sampling

switch opens and conversion begins, the ADC output is inaccurate.

The rate of decay of the sampling error and its nonlinearity over input voltage are highly sensitive to source

impedance. In other words, for larger values of source impedance, the sampling error decays more slowly over

time, resulting in greater residual error at the end of the sampling window that is also more non-linear over the

ADC input voltage range. Nonlinearity in the ADC input translates to nonlinearity or harmonic distortion in the

ADC output. Harmonic distortion degrades ADC resolution and translates to a decrease in the ADC’s effective

number of bits (ENOB). Therefore, driving the ADC input with a low-impedance source is critical for conversion

accuracy.

In addition to keeping source impedance as low as possible, TI recommends the following measures for

minimizing input sampling error and harmonic distortion associated with the TLV2553 while operating the device

at maximum 200-KSPS throughput:

•

For AC inputs, the maximum input signal frequency on all channels should be limited to well less than the

maximum Nyquist rate of 100 kHz. Figure 40 shows how ENOB degrades as input frequency increases.

Figure 40. ENOB as a Function of Input Signal Frequency

•

For DC inputs, ensure that there are no large step-function changes (greater than VREF / 4) between

successive input channels in the scanning order at the highest throughput. If possible, it is advisable to scan

the input channels so that the difference in the DC voltage levels between any two successive channels is

minimized to ensure 12-bit sampling accuracy. For larger voltage changes between channels, higher

accuracy can be achieved by reducing the throughput.

26

Submit Documentation Feedback

Copyright © 2001–2015, Texas Instruments Incorporated

Product Folder Links: TLV2553