THS4521 datasheet(34/53 Pages) TI2

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10.2.3 Application Curves

Figure 10-8 and Figure 10-9 show the gain response and the noise results of the circuit shown in Figure 10-7.

Figure 10-7 shows a place for a differential input capacitor (shown as 100 fF) but is not used for the simulation

results shown in this section. Results in Figure 10-8 illustrate a flat Butterworth filter response at the output

nodes going to the ADC. Obtaining the SNR to the ADC input pins, and assuming an 8-VPP full scale (2.83

VRMS), gives the result of Figure 10-9. The 116-dB SNR and 13-µVRMS total noise shown in Figure 10-9 does not

limit the performance for any SAR application.

Frequency (Hz)

-90

-84

-78

-72

-66

-60

-54

-48

-42

-36

-30

-24

-18

-12

-6

10k

100k

10M

D101

Figure 10-8. Gain Plot for a 170-kHz Butterworth

Filter

Frequency (Hz)

115

120

2.5

125

130

7.5

135

140

12.5

145

10k

100k

10M

D102

SNR

Total Noise

Figure 10-9. Signal-to-Noise Ratio and Total Noise

Plot

11 Power Supply Recommendations

The THS4561 is principally intended to operate with a nominal single-supply voltage of 3 V to 12 V. Supply

voltage tolerances are supported with the specified operating range of 2.85 V (10% low on a 3-V nominal supply)

and 12.6 V (5% high on a 12-V nominal supply). Supply decoupling is required, as described in Section 8.7. Split

(or bipolar) supplies can be used with the THS4561, as long as the total value across the device remains less

than that specified in Section 7.3.

Using a negative supply to deliver a true swing to ground output when driving SAR ADCs can be desired.

Although the THS4561 quotes a rail-to-rail output, linear operation requires approximately 200-mV headroom to

the supply rails. One easy option for extending the linear output swing to ground is to provide the small negative

supply voltage required using the LM7705 fixed –230-mV, negative-supply generator. This low-cost, fixed,

negative-supply generator can accept a 3-V to 5-V positive supply and provides a fixed –230-mV supply for the

negative power supply. Using the LM7705 provides an effective solution, as discussed in the Extending Rail-to-

Rail Output Range for Fully Differential Amplifiers to Include True Zero Volts reference guide.

THS4561

SBOS874D – AUGUST 2017 – REVISED FEBRUARY 2021

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