DN323 - New Instrumentation Amplifiers Maximize Output Swing on Low Voltage Supplies

New Instrumentation Amplifiers Maximize Output Swing
on Low Voltage Supplies – Design Note 323
Glen Brisebois
INTRODUCTION
Instrumentation amplifiers suffer from a chronic output
swing problem, even when the input common mode
range and output voltage swing specifications are not
violated. This is because the first stage of an instrumentation amplifier has internal output voltages that can
clip at unspecified levels. The clipping itself is invisible
to the user, but it affects the output swing adversely,
usually causing a gain reduction and thus an invalid
result. The new LTC ®6800 and LT®1789-10 both solve
this output swing problem, but in two extremely different ways. The LTC6800 incorporates a flying capacitor
differential level shifter followed by a rail-to-rail output
autozero amplifier. The LT1789-10 is a more classical
three op amp instrumentation amplifier with the twist
that it takes gain in the final stage.
A CLEARER PICTURE OF THE PROBLEM
Figure 1 shows the classical three op amp instrumentation amplifier (IA) topology. Assume that the op amps
involved can common mode to VS – and have rail-to-rail
output stages. This would normally mean that the inputs
can be anywhere from VS – to about a volt from VS+ and
that the output can be anywhere within the supply rails.
+
R
–
RG
VCM
RF
R
RF
R
G=1+
+
2RF
RG
VOUT
A3
–
VDM
2
–INPUT
R
–
DN323 F01
A2
+
VS–
FIRST STAGE
What can be misleading about this particular error mode
is that the gain does not fall to zero, so bench validation
tests performed in haste may not catch the problem.
The gain is reduced, but there is still a partial signal
gain path maintained by A1 and A3 (until A3 clips, of
course). Figure 2 shows the entire range of valid output
swing vs input common mode for an IA similar to that
described above powered on a single 5V supply1. Note
that with the inputs near ground or near 4V, the IA has
essentially no valid output swing!
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners. 1This plot is actually taken from an LT1789-1 and incorporates
improvements near ground due to a level shifting input PNP stage.
VCM(HIGH),
THIS OP CLIPS
A1
VDM
2
For example, assume that the IA is powered on a single
5V supply (VS+ = 5V, VS – = 0V), set for a gain of 3
(RG = RF ), and that its inputs are centered at VCM =
0.5V. Now, as the differential input voltage is increased
around the 0.5V common mode, the output voltages
of amplifiers A1 and A2 split apart as well. Note what
happens, though, when the differential input voltage
(VDM) reaches 1/3V. At that point, the output of A1
goes to 1V and the output of A2 goes to 0V, where it
is clipped by the negative supply rail. This happens in
spite of the fact that no specified input common mode
range or specified output swing has been exceeded.
VCM(LOW),
THIS OP CLIPS
5
VALID OUTPUT VOLTAGE (V)
VS+
+INPUT
But analysis of the circuit shows that these conditions
are not sufficient to ensure a valid output.
TA = 25°C
VS = 0V, 5V
4
3
G=1
2
G=2
1
G = 10
SECOND STAGE
0
Figure 1. Classical Three Op Amp Instrumentation
Amplifier. First Stage can Have Clipping Problems
Depending on VCM. This Reduces the Gain and Gives
a False Output Reading
10/03/323_conv
0
1
3
4
2
INPUT COMMON MODE VOLTAGE (V)
5
DN323 F02
Figure 2. Using Rail-to-Rail Output Op Amps Doesn’t
Guarantee Output Swing over Input Common Mode
THE SOLUTIONS
Figure 3 shows the same plot, this time for an LT1789-10.
Note the drastic improvement. A simplified schematic of
the LT1789-10 is shown in Figure 4. The PNP transistors
on the inputs serve to level shift the input voltages up
by one VBE, thus ensuring valid small-signal input and
output ranges (for A1 and A2) near VS –. But the real
key to the dramatic improvement in output swing is the
gain of 10 in the last stage. By taking gain in the last
stage, the outputs of the first stage are not required
to swing as much for a given overall gain setting and
desired output swing.
5
G = 10
VALID OUTPUT VOLTAGE (V)
4
G = 100
3
TA = 25°C
VS = 0V, 5V
2
The LTC6800 Solution
The LTC6800 achieves similar immunity from the output
swing vs input common mode problem, but in a completely different way. The device incorporates a flying
capacitor differential level shifter followed by a very
precise autozero output op amp as shown in Figure 5.
The rail-to-rail output op amp is gain configurable in
the conventional 2-resistor way, and follows the usual
noninverting gain equation G = 1 + RF/RG.
Figure 6 shows the valid output swing vs input common mode for the LTC6800. In a gain of 1, the output
validity is clipped to about 3.5V by the input common
mode range of the op amp A1. Elsewhere in the plot,
the ramp-like limitation characteristics are due to the
input referred voltages at the rail-to-rail input switches
and capacitors clipping at the supply rails. Like the
LT1789-10, the LTC6800 performance represents a
dramatic improvement over the classical results of
Figure 2.
INPUT CM LIMIT
ON 5V SUPPLY
1
8
V+
+IN
0
0
1
3
4
2
INPUT COMMON MODE VOLTAGE (V)
CS
–IN
DN323 F03
CH
(t 1+
5.7k
–
RG 1
+
V–
RG
R1
10k
R2
100k
5
V–
+
A3
–
5.7k
V+
–IN 2
–
V–
+
V–
R3
10k
DN323 F05
RF
Figure 5. The LTC6800 Block Diagram with External Gain
Set Resistors also Shown
V+
5 REF
RF
100k
RG 8
2RF
RG
A1
V + VB
V+
4
RG
+IN 3
VOUT
V–
RG
6
+
RF
100k
V–
REF
5
R4
100k
A2
VB
6 VOUT
7 V+
V–
DN323 F04
4 V–
G = 10
VALID OUTPUT VOLTAGE (V)
V
7
–
Figure 3. The LT1789-10 Gives Effective Rail-to-Rail
Output Validity over Almost the Entire Input Common
Mode Range
V
OUT
A1
2
+
R
G = 1+ F
RG
+
3
5
4
G=2
G=1
3
2
1
0
0
1
3
2
VCM(IN) (V)
4
5
DN323 F06
Figure 4. The LT1789-10 Block Diagram. PNP Inputs
Level Shift Away from VS–. Gain of 10 Around A3 Eases
Output Swing Requirements on A1 and A2
Data Sheet Download
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Linear Technology Corporation
Figure 6. The LTC6800 Output Swing vs Input Common
Mode. Drastic Improvement over Classical Architecture
For applications help,
call (408) 432-1900
dn323f_conv LT/TP 1003 316.5K • PRINTED IN THE USA
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