Interfacing Analog Signals to FlexAnalog FPAAs

App Note: AN303 (338 kB, 1pdf file):  FlexAnalog Interfacing Analog Signals

Introduction

FlexAnalogTM FPAAs such as the AN231 and AN241 use analog signals that are referenced to +1.5 V (VMR) and are limited to the range 0 to +3 V. This application note describes a number of methods for interfacing these FPAAs to signals that are biased at voltages other than +1.5 V and/or whose amplitude exceeds the range 0 to +3 V.

Input and Output Signals
First of all, it is necessary to explain in more detail the kind of signals that can be handled by the FPAA. The following discussion applies to AN231 and AN241.

All FlexAnalogTM FPAAs are single supply devices. That is to say, they work from a positive voltage supply (Vdd) and ground. There is no negative supply which means that the FPAA cannot handle signals that are negative, either on its inputs or its outputs. It is an obvious point, but worth stressing that the FPAA can only handle signals that are between ground and its positive supply. For the AN231 and AN241 that supply is +3.3V.

For this reason, the FPAA has an internal signal ground that is fixed at a positive voltage just below half the supply voltage. This signal ground is set to +1.5V. Note that the user does not need to supply this signal ground, it is generated within the FPAA and is output to a pin called VMR.

All the analog signals paths within and at the I/O of the FPAA are differential, consisting of two equal and opposite signals, both centered on VMR, both restricted to the range of ground and Vdd. The maximum signal that the FPAA can handle is
+/-3V, or 6V peak to peak. It is important to understand that this is differential amplitude, not a single-ended amplitude.

How is this maximum amplitude calculated?
If the signal (input or output) is at +3V, then this means that there is +3V on the positive side and 0V on the negative side (must be equal and opposite centered about VMR). When the signal is at -3V, then there is 0V on the positive side and +3V on the negative side.

To fully utilize the FPAA (and maximize the performance) in a system with single-ended analog signals, it is necessary to convert input signals to VMR referenced differential signals, and to convert the output VMR referenced differential signal into whatever form the user requires (typically ground referenced and single-ended).

This application note provides two simple methods for interfacing to FlexAnalogTM FPAAs using ground referenced single- ended signals. One applies to the input signals, the other to the output signals.

Signal Input Using the Rauch Filter FlexAnalogTM FPAAs provides an optional input OpAmp, available within IOs 1-4. This I/O OpAmp allows the user to construct a circuit, using only passive components, which performs four tasks: Level shifts an input signal bias voltage to VMR. Converts a single-ended signal to differential signal (if required). Performs a 2-pole (second order) low-pass anti-aliasing filter function on the input signal to reduce noise. It can amplify or attenuate an input signal to best match it to the input range of the FPAA. This circuit used is called a Rauch filter (or low-pass multiple feedback filter) and is shown in Figure 1.

The values of the components can be calculated from the required values of Fo (low-pass corner frequency), Q (the filter Q, use 0.707 for optimum flat response in pass-band), and G (the pass-band gain). The equations are shown below.

First, choose a convenient value of R1. Then we have:

R2 = R1 * G
R3 = R1 * G / (G + 1)
C1 = Q * (G + 1) / (G * Fo * R1 * 2 * π) C2 = 1 / (G * Fo * R1 * Q * 4 * π)

The signal applied to the Rauch filter can be single-ended or differential and centered on any voltage. It can also be any amplitude provided the gain G is set to provide the correct amplitude at the FPAA pins. The corner frequency Fo should be set to a value just above the working range of the input signal. If a ground referenced single-ended signal is input to the Rauch, then the other input should be tied to ground.

Let us consider an example: suppose we wish to input a single-ended ground referenced signal of +/-10V to the FPAA. Also, let us suppose that this signal ranges in frequency up to 10kHz. We need to set the parameters as follows:

G = 0.3 (3V max amplitude divided by 10V input amplitude)
Q = 0.707 (optimum flat response)
Fo = 12kHz (just above the frequency range)

This gives component values of:
R1 = 10k (choose this) R2 = 3k
R3 = 2.3k C1 = 4.1n C2 = 3.1n
Obviously, the user will choose preferred component values that are as close as possible to these values.

The user can then reverse calculate the filter parameters from the following equations:

G = R2 / R1
Fo = 1/(2 * π * R2) * sqrt[(R1+R2)/(2 * C1 * C2 * R1)]
Q = sqrt[C1 * R1/(2 * C2 * (R2+R1))]

For example: the user might choose the following component values:
R1 = 10k R2 = 3.3k R3 = 2.2k C1 = 4.7n C2 = 3.3n
In this case, we can reverse calculate the filter parameters to get:
G = 0.33
Fo = 9.99kHz Q = 0.73

Signal Output Using an Opamp

Figure 2 shows how a single opamp can be used to not only level-shift the FPAA output but also do a differential to single- ended conversion. Furthermore, this circuit can be used to amplify or attenuate the FPAA output by any desired amount.

The gain of this circuit is given by:
Gain = Rf / Ri

Note: The circuit in Figure 2 shows the level shifting of the FPAA output to ground, and it is the resistor Rf connected to ground that provides the reference for the level-shifted signal. However, this circuit can be used to level-shift the FPAA output to any desired voltage simply by connecting the resistor Rf to that voltage instead of ground.

In addition to level-shifting and amplifying/attenuating the FPAA output, the circuit can also be turned into a simple one- pole low-pass filter by adding capacitors in parallel with the two resistors marked Rf.

The corner frequency of this filter would be given by:
Fo = 1 / (2 * π * Rf * C)