140dB Range (1nA to 10mA)
SGM834A
Logarithmic Current-to-Voltage Converter
APPLICATION INFORMATION (continued)
The following equation can be used for calculating the
resistors when a lower intercept point is desired:
VLOG Output Conditioning
The on-chip buffer amplifier and voltage reference can
be used for adjusting the output intercept point and the
scaling needed to match the desired signal level of the
downstream process. Figure 10 shows the circuits that
can be used for lowering or raising the intercept point.
10
RZ
IPD
IZ
RLOG
VOUT = G× V ×
×log
+ V
×
Y
REF
RZ + RLOG
RLOG + RZ
(6)
RA
where:
and RLOG = 5kΩ.
G = 1+
VOUT
RB
RZ
VREF
2V
RA
-
Usually it is more useful to raise the intercept point.
Note that raising the intercept point reduces all output
values. Figure 10 (b) shows how the intercept point is
raised by placing the RC between the BFNG and VREF
pins. This arrangement raises the BFNG voltage (with
zero input signal) and pushes the VOUT to lower
values. Note that with the RC connected to the BFNG
pin, the gain (G) is also affected, but it can be
compensated and re-adjusted by RA and RB. Use the
following equation for calculating the resistors (Figure
10 (b)):
BFNG
BFIN
Buffer
ILOG
RB
VLOG
VLOG
AGND
5kΩ
(a) Lowering the intercept point
VOUT
RC
VREF
2V
RA
-
BFNG
Buffer
ILOG
BFIN
RB
10
IPD
IZ
RA ||RB
VLOG
5kΩ
(7)
VLOG
AGND
VOUT = G V ×log
- V
×
Y
REF
RA ||RB + RC
RA
RA ×RB
RA + RB
(b) Raising the intercept point
where:
and
RA ||RB =
G = 1+
RB ||RC
.
Figure 10. Adjusting the Intercept Point and Scaling
Table 2. Some Examples for Raising the Intercept
Table 1 lists some examples for selecting resistors for a
few slope scales values while the intercept point is
reduced from the IZ = 100pA. VY in mV/dec is called the
slope voltage.
VY (mV/dec)
300
IZ (nA)
10
RA (kΩ)
7.5
RB (kΩ)
37.4
130
RC (kΩ)
24.9
18.2
25.5
16.2
13.3
24.9
16.5
12.4
300
100
10
8.25
10
400
16.5
25.5
36.5
12.4
16.5
20.0
400
100
500
10
9.76
9.76
12.4
12.4
11.5
Table 1. Some Examples of Lowering the Intercept Point
400
VY (mV/dec)
200
IZ (pA)
1
RA (kΩ)
20.0
10.0
3.01
10.0
8.06
6.65
11.5
9.76
8.66
16.5
14.3
13.0
RB (kΩ)
100
100
100
12.4
12.4
12.4
8.2
RZ (kΩ)
25
500
500
100
500
200
10
50
1
50
500
200
165
25
300
300
10
50
1
50
300
165
25
400
400
10
50
1
8.2
50
400
8.2
165
25
500
8.2
500
10
50
8.2
50
500
8.2
165
SG Micro Corp
www.sg-micro.com
FEBRUARY 2022
17
SGMICRO [ Shengbang Microelectronics Co, Ltd ]
