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TLV2553

SLAS354C – SEPTEMBER 2001 – REVISED SEPTEMBER 2015

www.ti.com

Typical Application (continued)

9.2.2 Detailed Design Procedure

Good dynamic performance while the ADC is multiplexing inputs at maximum sampling rate requires low source

impedance on the input channels being addressed. To make the input source impedance less sensitive to line

inductance, especially in cases where the signal sources may be located far away from the ADC, it may be

necessary to use operational amplifier buffers located close to the ADC input pins.

The procedure for estimating the maximum tolerable value of input source impedance on a given channel for

achieving the desired ENOB (for example ENOB > 11.5) in a multiplexed application is as follows:

1. Using a low impedance signal source, apply a full-scale sinusoidal signal of suitably low frequency to the

ADC input channel of interest, CHx.

2. Using a second low impedance source, apply a full-scale sinusoid that has the same frequency as the signal

on CHx but is 180

˚ out-of-phase, to a second ADC input channel, CHy, that will serve as the control element

in the experiment.

3. Initiate conversions with the ADC continuously multiplexing between CHx and CHy in each conversion cycle.

4. Rearrange the output data by channel, and for each of the two channels, compute SINAD from its FFT and

estimate ENOB for that channel as ENOB = (SINAD[dB] – 1.76) / 6.02.

5. Increase the series resistance on CHx by a discrete amount and repeat steps 1 through 5 until the ENOB of

CHx has degraded sufficiently relative to CHy (which should remain unchanged).

The external 1-nF decoupling capacitors (recommend C0G/NP0 type for constant capacitance versus voltage) on

the input channels are required for supplying the instantaneous change in the ADC’s load current demand during

the sampling phase after an input channel is selected. In other words, the decoupling capacitor effectively

reduces the output impedance of the source at high frequencies.

Similarly, the reference pin also requires decoupling for low-output impedance at high frequency. However, the

larger magnitude of reference pin load currents during the ADC conversion phase necessitates a decoupling

capacitor of a much higher value. The extra ESR (2.5

Ω) is required for stabilizing the OPA320 output as it drives

the 10-

μF load.

The OPA320 is a wide-band, low-noise, low-power operational amplifier that is unity gain stable and can operate

on a single 5-V system supply while supporting rail-to-rail signal swing at its input and output. These properties

make it an ideal choice for being used as a high-precision (stable, low-noise) reference buffer that has enough

loop gain over frequency to support low-output impedance over a wide bandwidth.

9.2.3 Application Curve

Figure 42. ENOB as a Function of Input Source Impedance

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