Product Overview
©
2009 Microchip Technology Inc.
DS51834A-page 9
1.4.3
DUT Bias Voltage Inputs
Figure 1-3 shows the basic DUT biasing circuitry, except the input pins which have
already been discussed (V
CMX
= 0V).
FIGURE 1-3:
DUT Bias Circuitry.
Lab power supplies are connected to V
DDX
and V
SSX
. The resistors R
29
, R
30
, R
37
and
R
38
, along with the capacitors C
13
, C
14
, C
21
and C
22
, minimize crosstalk from the other
op amps on the board. Since the MCP651’s quiescent current is between 3 mA and
9 mA, the actual power supply voltages (V
DDI
and V
SSI
) are different by 30 mV to
90 mV. I
DD
and I
SS
can be calculated as (I
SS
is negative):
EQUATION 1-3:
The DUT’s V
CAL
pin sets its internal common mode voltage (V
CMX
of the MCP651)
when it is in calibration mode. Thus, the DUT’s offset (V
OS
) is small when V
CMX
(0V on
this board) is equal to V
CALX
.
The RCAL potentiometer (POT or R43), with the resistors R
42
and R
43
, sets V
CALX
.
The values chosen allow the POT to cover the specified V
CALX
range, and a little more.
Connecting a voltmeter to V
CALX
makes it possible to set the POT accurately. Notice
that it is also possible to drive V
CALX
with an external voltage source (the wiper should
be, but doesn’t have to be, at mid-range).
V
OUTX
is set, as previously explained, to be equal to V
COX
by the integrator in the PI
control loop. If V
COX
is at or beyond V
SSI
or V
DDI
, then the loop forces V
OUTX
to be
railed at the corresponding supply voltage.
The load resistor (R
L
or R
10
) is biased to the externally supplied voltage V
LX
. V
LX
is
usually set to mid-supply. The V
LX
connection can be left open, which minimizes the
loading on V
OUTX
(about 40 k
Ω
).
R
44
R
43
C
25
R
42
2.2 k
Ω
10 k
Ω
150
Ω
R
37
4.49
Ω
V
DDX
100 nF
DUT
V
CAL
U
1
V
SS
V
DD
V
OUT
V
OUTX
C
21
10 µF
V
DDI
R
38
4.49
Ω
V
SSX
V
SSI
I
DD
R
10
1.00 k
Ω
V
LX
R
29
4.49
Ω
R
30
4.49
Ω
C
22
10 µF
C
13
100 nF
C
14
100 nF
I
SS
V
CALX
I
DD
V
DDX
V
DDI
–
10
Ω
---------------------------------
=
I
SS
V
SSX
V
SSI
–
10
Ω
----------------------------
=
