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

advertisement
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
+
VDM
2
1This
R
–
RG
VCM
RF
R
RF
R
G=1+
+
VOUT
A3
–INPUT
R
–
DN323 F01
A2
+
VS–
FIRST STAGE
5
2RF
RG
–
VDM
2
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!
,LTC and LT are registered trademarks of Linear Technology Corporation.
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
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
VALID OUTPUT VOLTAGE (V)
VS+
+INPUT
that the output can be anywhere within the supply rails.
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
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
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
www.BDTIC.com/Linear
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
1
3
4
2
INPUT COMMON MODE VOLTAGE (V)
0
5
CH
( )
G = 10 • 1+
+IN 3
REF
5
R1
10k
+
V–
RG
R2
100k
A1
RG 8
V–
+
RF
100k
A3
–
5.7k
V+
–IN 2
–
V–
V–
RG
6
4
+
V–
R3
10k
R4
100k
A2
6 VOUT
7 V+
VB
DN323 F05
RF
5
5 REF
V + VB
V+
VOUT
Figure 5. The LTC6800 Block Diagram with External Gain
Set Resistors also Shown
V+
V–
DN323 F04
G = 10
VALID OUTPUT VOLTAGE (V)
–
RG 1
2RF
RG
7
–
RG
5.7k
OUT
A1
2
V+
RF
100k
V–
CS
–IN
Figure 3. The LT1789-10 Gives Effective Rail-to-Rail
Output Validity over Almost the Entire Input Common Mode
Range
R
G = 1+ F
RG
+
3
DN323 F03
V+
V+
+IN
0
4
G=2
G=1
3
2
1
0
0
4 V–
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
Figure 6. The LTC6800 Output Swing vs Input Common
Mode. Drastic Improvement over Classical Architecture
Data Sheet Download
http://www.linear.com search for LTC6800
http://www.linear.com search for LT1789-10
Linear Technology Corporation
For applications help,
call (408) 432-1900, Ext. 2156
dn323f LT/TP 1003 316.5K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
www.BDTIC.com/Linear
FAX: (408) 434-0507 ● www.linear.com
 LINEAR TECHNOLOGY CORPORATION 2003