Analog switch capacitance compensation
M0RZF
Overcoming analog switch channel capacitance for filter switching
1. The Problem
There are several ways to switch front end filters for transceivers at HF frequencies:
• Mechanical relays
• PIN diodes
• NMOS/CMOS switches
• pHEMT/UltraCMOS
Typically 8 bandpass filters are needed to cover 1.8MHz to 30MHz.
Mechanical relays are low loss, but bulky and expensive. PIN diodes are capable of good
performance, but need quite a high bias current, and a lot of untidy little passive parts.
For the mobile comms market, pHEMT (pseudo-high electron mobility transistor) was developed to
make switches and other parts at higher frequencies than normal CMOS switches. UltraCMOS from
Peregrine semiconductor is a similar technology, used in the Elecraft KX3 and other radios. They
sell multiplexers but the lower frequency limit of that type of technology is doubtful. The
multiplexers are also in difficult packages, like ball grid arrays.
Traditional CMOS analog switches have a trade-off between channel capacitance (bandwidth) and
on resistance. A larger area device has lower resistance, but more area, which reduces bandwidth.
Bus switches such as the FST series from Fairchild semi, have 7 ohms or more on-resistance. By
comparison something like the ADG708 from Analog Devices has <3 ohms resistance, but nearly
100pF of channel capacitance. So it seems we can’t win.
2. Bandwidth is relative
Thinking about the actual requirement, we don’t need
wide bandwidth. The channel capacitance of the
ADG708 (other switches were considered) can be
absorbed into the filter design. Consider an approximate
equivalent of the ADG708, shown left.
Approximating further, its basically a 97pF capacitor in
parallel with the load we are driving (assume 50 ohms)
on the far side. So our filter sees 50R in parallel with
97pF. That’s why the spec sheet quotes 55MHz as the -3dB bandwidth.
If a filter is designed for 50 ohm specification, it ends up with a bad response:
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Analog switch capacitance compensation
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The problem is to change this mess to a nice flat response. One way is to design the filter with a
parallel capacitor on the input and output. The channel capacitance is then simply subtracted. The
problem with that is the bias voltage of the analog switch.
The CMOS switch channel cannot be grounded, because the signal has to swing around a voltage
like 2.0V. Having a parallel input filter means an inductor. This shorts to ground, unless a coupling
capacitor is added, which means 16 extra capacitors in an 8 channel bank.
3. Mesh coupled filter design method
Bandpass filters with capacitor coupled inputs are better because bias for the switches is not
shorted to ground. But there is the problem of how to design with a reactive non-50 ohm
terminating impedance. The solution is to do a parallel to series circuit conversion with the 50R
and 97pF. The formulas for doing that are in a textbook.
But for bands <5MHz the channel capacitance is small relative to the filter values. The capacitance
can be ignored really.
Designing step by step:
1. Start with your centre frequency and bandwidth.
2. Calculate the ‘Q’ factor at the centre frequency for R ll C.
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Analog switch capacitance compensation
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3. From Q factor calc Rs
4. Then calc Cs
5. Design using ELSIE for “mesh coupled bandpass” and put in the Rs previously calculated for the
terminating impedance.
6. Use capacitors in series formula to combine the Cs value with the series capacitor value found
by ELSIE.
To do the non-ELSIE bits, I prepared an embedded spreadsheet. The impedance outside the filter
is assumed to be 50 ohms. Cp is the channel capacitance of the switch. Fc is the centre frequency
of the filter. Rs is the equivalent series resistor and Cs the series capacitance.
Switch parallel C
1.00E-10
BAND
5-8MHz
9-12MHz
13-18MHz
20-23MHz
24-30MHz
45-55MHz
Centre freq.
6.00E+06
1.00E+07
1.50E+07
2.15E+07
2.70E+07
5.10E+08
Q at Fc
1.88E-01
3.14E-01
4.71E-01
6.75E-01
8.48E-01
1.60E+01
Series Rs
48.28
45.51
40.91
34.34
29.08
0.19
Series Cs
2.91E-09
1.11E-09
5.50E-10
3.19E-10
2.39E-10
1.00E-10
ELSIE series C
4.00E-10
1.37E-10
1.00E-10
5.20E-11
6.80E-11
2.00E-11
Final series C
4.64E-10
1.56E-10
1.22E-10
6.21E-11
9.50E-11
2.50E-11
To use the spreadsheet, change the blue cells to suit the values you want. Red values are
calculated outputs. Parallel capacitance has been entered as all the same just to make the sheet
layout work.
4. Proving it works
The 24-30MHz filter circuit including the terminations becomes:
Running the complete equivalent circuit for a 12m/10m bands filter through a simulator returns a
smooth (and low loss) response. The -6dBV loss shown is an artifact of the simulation, it’s the
flatness of the trace that proves it’s working. The bottom trace shows the reflection from the filter,
as it is a measurement of the input voltage. The complete 8 channel filter bank will be tested on
hardware during 2014. Of course there are two problems with my neat scheme:
1. At 50MHz the impedance drops to 15ohms, and inductor resistance (‘Q’) makes a filter too
lossy, rather defeating the object. Using a parallel input filter topology with decoupling caps
may work…
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Analog switch capacitance compensation
M0RZF
2. The input impedance of a following QSD stage is not 50ohms, especially across a wide
bandwidth.
M0RZF
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Feb 2014