THS4031_16 Datasheet, PDF (26/47 Pages) Texas Instruments – THS403x 100-MHz Low-Noise High-Speed Amplifiers

THS4031, THS4032
SLOS224H – JULY 1999 – REVISED JUNE 2016
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The TS5A3159 is a single-pole, double-throw (SPDT) analog switch that is designed to operate from 1.65 V to
5.5 V. The device offers a low ON state resistance and an excellent ON resistance matching with the break-
before-make feature to prevent signal distortion during the transfer of a signal from one channel to another. The
device has an excellent total harmonic distortion (THD) performance and consumes low power. The TS5A3359 is
a single-pole, triple-throw (SP3T) version of the same switch
10.2.2.1 Selection of Multiplexer
Figure 61 shows an equivalent circuit diagram of one of the channels of a multiplexer. C_S is the input
capacitance of the channel; C_D is the output capacitance of the channel. R_ON is the resistance of the channel
when the channel is ON. C_L and R_L are the load capacitance and resistance, respectively. V_IN is the input voltage
of the source; R_S is the source resistance of the source. V_OUT is the output voltage of the multiplexer.
Figure 61. Multiplexer Equivalent Circuit
To improve settling time, the values of R_S, R_ON, C_S, C_D, and C_L need to be smaller, and the value of R_L should
be large.
For TS5A3159:
• R_S = 1 Ω
• C_S = C_D = 84 pF
Considering
• R_S = 50 Ω
• C_L = 5 pF
• R_L = 10 kΩ
• T_RC (time constant) = 8.65 ns
For a 16-bit system, at least 18-bit settling is required. For 18-bit settling, the time required is (18 × ln2) × T_RC =
108 ns, which is better than 2 MSPS (500 ns). If the settling time is more than the conversion time of the ADC,
the output of the multiplexer does not settle to the required accuracy which results in harmonic distortion.
One more important parameter of a multiplexer is the ON-state resistance variation with voltage. This also affects
distortion because R_ON and R_L act like a resistor divider circuit and any variation of R_ON with voltage affects the
output voltage.
10.2.2.2 Signal Source
The input signal source should be a low-noise, low-distortion source with low source resistance. As discussed in
the earlier section, RS should be low to improve settling time. If the source is not a low-noise and low-distortion
source, a passive band-pass filter can be added to improve the signal quality as shown in Figure 60.
10.2.2.3 Driving Amplifier
The driving operational amplifier (OPA3 in Figure 60) in this application needs to have good slew rate and
bandwidth along with low noise and distortion. The input of the operational amplifier may see a maximum step of
4 V because of MUX switching. So, even if the signal bandwidth is low, the driving amplifier needs to settle from
0 V to 4 V (or 4 V to 0 V) within one ADC sampling frame. When selecting the operational amplifier, one must
ensure that it can settle from 0 V to 4 V (or from 4 V to 0 V) within the ADC sampling time (in this case 500 ns).
The amplifier used for driving the ADC is the THS4031. The operational amplifiers (OPA1, OPA2 in Figure 60)
used before the MUX is for signal conditioning. These operational amplifiers need to have low noise and
distortion.
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