Preliminary price estimation for the voltmeter's analog front end

Here is a preliminary price estimation for the analog front end of the digital voltmeter.

The BOM includes only the most expensive and important parts, but not :

- The micro controller. 

- The power supply.

- Mechanical parts like banana terminals or case. 

- Passive components except the matched resistors for the voltage divider.

The price are for single quantities, without taxes and shipping taken from www.digikey.com site.

Component name	Description	pcs	Price, USD
LTC2057HVIMS	Input, switch and comparator op-amps	4	20
LT5400BCMS8E-8	Quad matched resistors	1	7.46
ADG5419BRMZ	Analog switch	1	4.7
MAX990	Comparator for the auto voltage range	1	1.62
MCP1501-20	Comparator Vref 2.048V	1	0.78
MCP6062	Dual opamp for comparator Vref	1	0.82
LT1001ACN8	ADC buffer	1	6.53
LTC2440CGN	24 bit ADC	1	11.18
LTC6655BHLS8	Voltage reference 5V	1	14.92
74HC4053PW	3x SPDT	1	0.41
MMBT3904	NPN transistors	3	0.48
Total price, without tax and shipping			68.9

Additional cost should be added for the PCB. If all components can fit in the 50mm x 50mm PCB, it will cost ~10-12 USD for 4 layer PCB with ENIG finish.

Final breadboard tests

I connected all small breadboards for the final tests and got big mess of wires:

The noise when inputs of the LTC2057 are shorted was in the 10-12uV p-p range which is ok if take in consideration the used voltage reference specs and the buffered op-amps (LTC2052) in the auto-range breadboard.

Unfortunately during the noise tests, I saw a voltage drift which is not acceptable. It is always positive and can reach 1-2 mV in period of 30-60 min:

For now I suspect the front end LTC2057 op-amp, because if I connect the voltage source direct to ADC driver or ADC itself, I do not get such drift. So I have to try one or more op-amps to resolve this issue.

Update:
I used 1.5 AAA battery when I saw the voltage drift. I measured with a Keithley 2002 and got similar drift again, so it is not from the LTC2057, but from the battery. I do not have yet explanation for this fact.

Breadboarding automatic voltage range

The proof of concept automatic voltage range is ready and it is working as expected.

The automatic voltage range is based on voltage divider, window comparator and single-pole, dual-throw (SPDT) switch. The middle point of voltage divider is connected to a window comparator which triggers input signal to the SPDT when the voltage is greater then fixed positive and negative threshold. In result the SPDT switch connects middle point of the voltage divider to the ADC buffer. When the voltage is less then the threshold, SPDT switch connect directly the output of the input buffer to the ADC buffer.

I made a breadboard with the following components:

• Input buffer LTC2057.
• Voltage divider with LT5400-3 (quad matched resistor network). This is the 100K/10K variant, but the final should be 9K/1K (LT5400-8) for 1:10 ratio and lower resistor noise.
• MAX990 as window comparator. 
• ADG5419 SPDT switch. Here I used IC solution, but pair of P and N-channel MOSFET can be used too for cost reduction. 
• MCP1501 voltage reference 2.048V with MCP6061T for reference negative voltage.  The voltage reference was divided with the spare resistors from the LT5400-3 which is the stable voltage threshold +/- 0.2048V for the window comparator.  
• LTC2052 : 2 additional op-amp buffers for input of the SPDT switch, one for the window comparator input and one for buffering the input of the negative voltage threshold.  

When the input voltage is less than 2.048V and greater than -2.048V, the comparator output is in a low logical level and the SPDT switch connects direct the LTC2057 to the ADC buffer. When the input voltage is greater than 2.048V or less than -2.048V, the comparator output is in high logical level and the SPDT switch connect the middle point of the voltage divider to the ADC buffer.

To use low cost, low voltage window comparators, the middle point of the voltage divider is connected to comparators. In this case the second pair of LT5400 resistors are used for decreasing the comparator's voltage reference.

The reference voltage of 2.048 was selected, because the SPDT switch needs time to switches between input buffer and voltage divider. If this time is too long, the ADC buffer and ADC inputs can be damaged by input voltage if power supply of the buffer is less then the input voltage. The maximum input voltage of the LTC2440 ADC is now 2.5V with the dual supply schematics. The rising slew rate of the LTC2057 is typically 1.3V/uS, which means that maximum time for switching should be no more then 0.347uS [(2.5-2.048)/1.3].

Picture of the breadboard and the power supply is shown below. This is the most density breadboard which I had ever made. From left to the right are placed : LTC2057, LT5400-3, ADG5419, MAX990, MCP1501 and MCP6061T. There was no space left for the ADC input buffer (LT1001A) so I have to place it in the ADC breadboard later.

The picture above is before to place additional op-amp buffers for the SPDT switch and for the comparator. The reasons behind this I found during testing functionallity of the schematics:

- If one of the switch is switched off, there is a float voltage which influence the input of the voltage comparator. In result when the higher or lower voltage than the comparator threshold is applied, the switch does not switches at all.

- During the noise test I found that input of the MAX990 comparators are too noisy, so adding one more low noise op-amp in front of the comparators resolved the issue. 

I used the LTC2052 quad op-amps which I had in stock from the ohmmeter schematics. Note that the maximum supply voltage for the op-amp connected directly after input buffer must be no less then the expected measured voltage.   

The schematics can be found in the git repository. Here is a screenshot: