ZHCSES1B March 2016 – October 2018 ADS1282-SP
PRODUCTION DATA.
Figure 26 shows a diagram of the input multiplexer.
ESD diodes protect the multiplexer inputs. If either input is taken less than AVSS – 0.3 V or greater than AVDD + 0.3 V, the ESD protection diodes may turn on. If these conditions are possible, external Schottky clamp diodes and/or series resistors may be required to limit the input current to safe values (see the Absolute Maximum Ratings).
Also, overdriving one unused input may affect the conversions of the other input. If overdriven inputs are possible, TI recommends clamping the signal with external Schottky diodes.
The specified input operating range of the PGA is shown in Equation 2:
Absolute input levels (input signal level and common-mode level) should be maintained within these limits for best operation.
The multiplexer connects one of the two external differential inputs to the preamplifier inputs, in addition to internal connections for various self-test modes. Table 2 summarizes the multiplexer configurations for Figure 26.
MUX[2:0] | SWITCHES | DESCRIPTION |
---|---|---|
000 | S1, S5 | AINP1 and AINN1 connected to preamplifier |
001 | S2, S6 | AINP2 and AINN2 connected to preamplifier |
010 | S3, S4 | Preamplifier inputs shorted together through 400Ω internal resistors |
011 | S1, S5, S2, S6 | AINP1, AINN1 and AINP2, AINN2 connected together and to the preamplifier |
100 | S6, S7 | External short, preamplifier inputs shorted to AINN2 (common-mode test) |
The typical on-resistance (RON) of the multiplexer switch is 30 Ω. When the multiplexer is used to drive an external load on one input by a signal generator on the other input, on-resistance and on-resistance amplitude dependency can lead to measurement errors. Figure 27 shows THD versus load resistance and amplitude. THD improves with high-impedance loads and with lower amplitude drive signals. The data are measured with the circuit from Figure 28 with MUX[2:0] = 011.