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I would like to use two transistor to switch on/off two paths to an ADC input so that only one path is active at a time. Is this a bad practice? I'm aware of analog switches but don't know if something that complex is necessary for my simple application. Is there any information about the resistance between the collector and emitter of NPN transistors? I want to make sure using the transistor as a switch for the analog signal doesn't mess up my analog readings in the ADC. Accuracy and resolution is important here (true 16-bit ADC). Would a particular type of transistor be preferable for this application if using transistors is viable at all? The AIN values can range from 0V to Vref on the ADC.

The circuit I'm considering is like this:

AIN1 ----- Transistor1 -----
                                             |
                                             |--------- ADC Input
                                             |
AIN2 ----- Transistor2 -----

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  • \$\begingroup\$ If accuracy and resolution are important, then it is better to go for analog switches. \$\endgroup\$ – nidhin Jul 2 '14 at 17:40
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    \$\begingroup\$ Just go for two ADCs if it's important - don't try and save $2 to find out it doesn't perform. What's it for anyway? \$\endgroup\$ – Andy aka Jul 2 '14 at 20:21
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For 16-bit performance you should use a high quality analog switch such as an ADG511, and buffer the output of the multiplexer with a precision op-amp that has low bias current (and offset voltage, and high open-loop gain). The analog switch includes low leakage, low Rds(on) switches and level shifting from logic.

Back in the dark ages, it was sometimes done to use bipolar transistors with controlled currents to the bases to switch precision analog signals (symmetrical transistors have very low Vce(on) under just the right conditions) but you do not want to do that. See for example this 1961 (!) reference, if you're interesting in paleolithic circuit design.

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  • \$\begingroup\$ I assume that it's also unsuitable to use a resistor to switch on a current sense resistor in an analog path for converting an analog current signal to a voltage signal? Which high-precision op amps would you recommend? \$\endgroup\$ – Pugz Jul 2 '14 at 18:02
  • \$\begingroup\$ @Pugz Depends a lot on supplies, max frequency etc. but AD8676 or OPA180 are pretty good (not rail-to-rail input, only output, but input CM range includes negative rail). \$\endgroup\$ – Spehro Pefhany Jul 2 '14 at 18:08
  • \$\begingroup\$ I assume that it's also unsuitable to use a transistor to switch on a current sense resistor in an analog path for converting an analog current signal to a voltage signal? \$\endgroup\$ – Pugz Jul 2 '14 at 18:16
  • \$\begingroup\$ Also, is the Ron value for an analog switch important at all if I've got a precision passive voltage divider (0.1% 100kohm resistors) after the analog switch but before the op amp (other than very slightly changing the series resistance in the voltage divider)? \$\endgroup\$ – Pugz Jul 2 '14 at 18:20
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    \$\begingroup\$ The leakage and op-amp bias current are more important, since 50K ohms source impedance at (say) 2.5V FS. means only 760pA leakage+bias current is one LSB. Ron affects linearity and offset if the op-amp has much bias current at DC, and does things at AC too. \$\endgroup\$ – Spehro Pefhany Jul 2 '14 at 18:27
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You should just use a CMOS analog switch if you're worried about accuracy. NPN and PNP tranistors are current-controlled devices, so you will need to deal with the bias current at the ADC input. The transistor will also have a relatively significant voltage drop (1.4V). MOSFET transistors do not have this issue, but you will need both a PMOS and an NMOS to pass a full-range signal. And you will need an inverter so both MOSFETS are controlled from the same logic input. CMOS switch chips have all of this integrated; all you need to do is supply power, the switch control signals, and the actual analog signals.

You should also consider the ADC input impedance. If you're trying to sample very quickly, adding a buffer amplifier might be advisable. Otherwise, a capacitor at the ADC input would help improve the accuracy. ADCs generally have a switched capacitor input and this can cause a bit of droop when the input sampling capacitor is charged. Adding an external cap on the pin will limit this.

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I think the other answers provide a good basis to start with. Here I'll bring in a few of the more subtle details.

The first question is, should I use a pass-gate or a transmission gate. A pass gate is a single transistor and a transmission gate is a complementary pair (PMOS and NMOS) driven with complementary signals.

In your case like others I'd suggest a transmission gate. But I will note that a pass-gate is often used in cases where extreme precision is required AND there is sufficient headroom. The reason is pretty simple, a transmission gate is simply two complementary pass-gates. A pass-gate will work from one rail all the way up to the other Vrail - Vth for NMOS and from the top rail all the way down to Vth above ground for PMOS. That means that for a transmission gate every thing is nice and sweet as long as the signal is at least Vth away from both rails, a pass-gate is sweet over a broader range but also doesn't approach the rails. On some ultra high resolution ADCs they use pass-gates, but also have to run higher rails.

The second thing you have to look for is the channel charge injection and overlap capacitance. When the control signal on the gate swings to turn off the transistor (or T-Gate) your output will bump a little. And this may be disruptive to your signal chain. Having two matched devices that are swing in complementary directions will help minimize this effect. Sometimes you have to hang dummy transistors to complement this signal.

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