*ADC Input Driver*
SINGLE-ENDED to
DIFFERENTIAL
CIRCUIT
ADCIN_SE_DIFF1.CR
Download the
SPICE file
The continuing story of input drivers brings us to the case where your
signal source is single-ended, but your ADC accepts a differential input.
For example, a position sensor that swings +/-10 V gets muxed to an ADC that expects a +/-4V differential
input centered around 2 V. What's the solution? The
Single-Ended to Differential Driver is your ticket to ride. To implement
this circuit, we actually use a couple of
Single-Ended to Single-Ended ADC
Drivers we covered earlier. By mirroring the gains and choosing the
right offset, you can properly feed your ADC's
differential input!
TWO OF A KIND
From our first look of the Single-Ended to Single-Ended driver
(assuming R2/R1 = R4/R3), we found that
**Vo = Vin+ ∙ ( R2/R1 ) + Vin-
∙ ( -R2/R1 ) + VREF**
This circuit gives you the choice of connecting your signal Vsig to
either Vin+ or Vin- (and grounding the other) providing you with either a
non-inverting or inverting amplifier. So let's take two of these
circuits and generate two outputs of opposite polarity.
**
Amplifier 1: Non-Inverting
**
**Vo+ = Vsig ∙ ( R2/R1 ) + VREF**
**
Amplifier 2: Inverting
**
**Vo- = Vsig ∙ ( -R2/R1 ) + VREF**
All that's left is choosing the proper gain/attenuation (K = R2 / R1) for
each amplifier and the offset (VREF).
HOW MUCH GAIN AND OFFSET?
The main thing to remember is that *each amplifier will provide half of
the required gain*. Back to our original challenge: a sensor generates a
±10 V single-ended signal will
ultimately feed an ADC that expects ±4 V
differential centered around 2 V. How much gain/attenuation do you need for
each amplifier?
K = ( ±4 V /
±10 V ) · 1/2
= 0.2 V/V
Choosing R2 = 10 kΩ, calculate R1 from
the gain equation K = R2/R1
R1 = R2 · K
= 10 kΩ · 0.2
= 2 kΩ.
For an offset of 2 V, simply set VREF to
this level.
TEST DRIVE
The SPICE file has two amplifiers XOP1 (with R11, R12, R13, R14) and XOP2
(with R21, R22, R23, R24) wired as a non-inverting and inverting amplifier,
respectively. The gains are set by
K = R2 / R1
= R12 / R11 = R14 / R13
= R22 / R21 = R24 / R23
HANDS-ON DESIGN
Start with
these initial
values: all Rs = 10k (K=1) and VREF = 0 V. Run a simulation of
ADCIN_SE_DIFF1.CIR. Plot the input V(1) and both the positive and negative
output, V(14) and V(24). In a separate window, plot the differential output
that the ADC would see,
V(14) - V(24). *You should see input/outputs swing ±10V
while the differential output swings ±
20V!* Cool, you get twice the swing from a differential output! But,
we're not there yet!
Now adjust the individual amplifier gain to 0.2 by changing R12 = R14 = R22 = R24 = 2 k.
Rerun the simulation and check the differential output. Yes, you've got the
desired +/- 4V differential swing. BUT, each output swings positive and negative! Not
good for an ADC that runs from a single +5 V rail.
Now set VREF = 2.0 V and rerun the SPICE file. Any improvement? Yes, each
output gets shifted by 2 V for a total swing between 0 and 4 V. Good news for the
ADC input! Did this shift have any effect on the differential output
V(14)-V(24)? More good news, VREF has no effect on the differential gain.
Looks like our circuit is ready to roll.
NEW SENSOR
HANDS-ON DESIGN
A new sensor has been dropped in your system. This device
swings ±1 V in response to a
± tilt angle. The ADC input expects
±5 V
differential centered around 2.5 V. Start with
these initial
values: all Rs = 10k (K=1) and VREF = 0 V. Change the source for 1 V peak
VS1 1 0 SIN(0V
+1VPEAK 1KHZ)
What values of gain resistors and VREF will make the ADC input happy?
MORE TOPICS
Here's around-up of the ADC driver topics to explore.
*Single-Ended Input to Single-Ended Output*
*Differential Input to Single-Ended
Output*
*
Single-Ended Input to Differential Output*
Differential Input to Differential Output
The last two functions require a couple of op amps. However, take a look at a single device - *
the Fully Differential Amplifier* - that
performs the functions.
SIMULATION NOTES
For a more detailed description of the op amp, see the Basic Op
Amp
Model.
For a quick review of subcircuits, check out Why Use Subcircuits?
Get a crash course on SPICE simulation at
SPICE Basics.
A handy reference is available at SPICE
Command Summary.
Browse other circuits available from the Circuit
Collection page.
SPICE FILE
Download the file
or copy this netlist into a text file with the *.cir
extension.
ADCIN_SE_DIFF1.CIR - SINGLE-ENDED TO DIFFERENTIAL ADC INPUT DRIVER
*
VS1 1 0 SIN(0V 10VPEAK 1KHZ)
VREF 10 0 DC 0V
*
* NON-INVERTING OUTPUT
R11 0 12 10K
R12 12 14 10K
R13 1 13 10K
R14 13 10 10K
XOP1 13 12 14 OPAMP1
*
* INVERTING OUTPUT
R21 1 22 10K
R22 22 24 10K
R23 0 23 10K
R24 23 10 10K
XOP2 23 22 24 OPAMP1
*
*
* SINGLE-POLE OPERATIONAL AMPLIFIER MACRO-MODEL
* connections: non-inverting input
* | inverting input
* | | output
* | | |
.SUBCKT OPAMP1 1 2 6
* INPUT IMPEDANCE
RIN 1 2 10MEG
* DC GAIN (100K) AND POLE 1 (100HZ)
* GBWP = 10MHZ
EGAIN 3 0 1 2 100K
RP1 3 4 1K
CP1 4 0 1.5915UF
* OUTPUT BUFFER AND RESISTANCE
EBUFFER 5 0 4 0 1
ROUT 5 6 10
.ENDS
*
.TRAN 0.01MS 2MS
*
.PLOT TRAN V(1) V(14) V(24)
.PRINT TRAN V(1) V(14) V(24)
.PROBE
.END
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